diff --git a/scripts/generate_pack.py b/scripts/generate_pack.py
index 78579542..665034e7 100644
--- a/scripts/generate_pack.py
+++ b/scripts/generate_pack.py
@@ -474,7 +474,9 @@ def _select_variants(
             required_only=required_only,
         )
     if regions:
-        region_index = region_mod.build_region_index(emu_profiles or {})
+        region_index = region_mod.platform_region_index(
+            config, emu_profiles or {}, target_cores
+        )
         region_drops = region_mod.resolve_region_drops(
             region_groups, region_index, regions
         )
@@ -3366,7 +3368,9 @@ def _manifest_selection(
         required_only=required_only,
         extras_out=extras,
     )
-    region_index = region_mod.build_region_index(emu_profiles)
+    region_index = region_mod.platform_region_index(
+        config, emu_profiles, target_cores
+    )
     drops = (
         region_mod.resolve_region_drops(region_groups, region_index, regions)
         if regions
diff --git a/scripts/packverify.py b/scripts/packverify.py
index c4eecdb0..087357f7 100644
--- a/scripts/packverify.py
+++ b/scripts/packverify.py
@@ -572,7 +572,9 @@ def verify_pack_against_platform(
 
     region_drops: set[str] = set()
     if regions:
-        region_index = region_mod.build_region_index(emu_profiles)
+        region_index = region_mod.platform_region_index(
+            config, emu_profiles, target_cores
+        )
         region_groups, _extra_dests = platform_region_groups(
             config,
             expected_systems,
diff --git a/scripts/region.py b/scripts/region.py
index d6336caa..9ae04912 100644
--- a/scripts/region.py
+++ b/scripts/region.py
@@ -125,6 +125,22 @@ def region_tag(requested: list[str]) -> str:
     )
 
 
+def platform_region_index(
+    config: dict, profiles: dict, target_cores: set[str] | None = None
+) -> dict[str, dict]:
+    """The region index of the emulators a platform runs, nothing else.
+
+    Built over every profile, an emulator foreign to the platform voided the
+    filter: Pizza Boy SC, an Android app, declares bios_CD_E.bin without a
+    region, so the Genesis Plus GX and PicoDrive tags never counted and
+    `--region us` kept both CD BIOSes in the RetroArch pack.
+    """
+    from common import resolve_platform_cores
+
+    cores = resolve_platform_cores(config, profiles, target_cores=target_cores)
+    return build_region_index({n: profiles[n] for n in cores if n in profiles})
+
+
 def build_region_index(profiles: dict) -> dict[str, dict]:
     """Build a region lookup from emulator profiles.
 
diff --git a/scripts/verify.py b/scripts/verify.py
index 1b46639e..40343b85 100644
--- a/scripts/verify.py
+++ b/scripts/verify.py
@@ -973,7 +973,9 @@ def verify_platform(
             target_cores=target_cores,
         )
         region_drops = region_mod.resolve_region_drops(
-            region_groups, region_mod.build_region_index(profiles), regions
+            region_groups,
+            region_mod.platform_region_index(config, profiles, target_cores),
+            regions,
         )
 
     # A destination two systems declare with two hashes ships one file, the
diff --git a/tests/test_region_platform_index.py b/tests/test_region_platform_index.py
new file mode 100644
index 00000000..a91bf96f
--- /dev/null
+++ b/tests/test_region_platform_index.py
@@ -0,0 +1,44 @@
+"""A platform's region filter listens to the emulators it runs only.
+
+The index was built over every profile: Pizza Boy SC, an Android app no
+frontend here runs as a core, declares bios_CD_E.bin without a region, which
+voided the [europe] tag Genesis Plus GX and PicoDrive give it, and
+`--region us` kept both foreign Mega-CD BIOSes in the RetroArch pack.
+"""
+
+from __future__ import annotations
+
+import sys
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(REPO_ROOT / "scripts"))
+
+import region as region_mod  # noqa: E402
+
+PROFILES = {
+    "gpgx": {"emulator": "GPGX", "type": "libretro", "systems": ["sega-mega-cd"],
+             "files": [{"name": "bios_CD_E.bin", "region": ["europe"]}]},
+    "pizza": {"emulator": "Pizza", "type": "standalone", "systems": ["sega-mega-cd"],
+              "files": [{"name": "bios_CD_E.bin"}]},
+}
+
+
+class OnlyThePlatformsCores(unittest.TestCase):
+    def test_a_foreign_untagged_entry_does_not_void_the_tag(self):
+        config = {"cores": ["gpgx"], "systems": {"sega-mega-cd": {"files": []}}}
+        index = region_mod.platform_region_index(config, PROFILES)
+        self.assertFalse(index["bios_CD_E.bin"]["has_untagged"])
+        self.assertTrue(region_mod.build_region_index(PROFILES)["bios_CD_E.bin"]["has_untagged"])
+
+    def test_every_platform_site_reads_it(self):
+        for module in ("verify.py", "generate_pack.py", "packverify.py"):
+            source = (REPO_ROOT / "scripts" / module).read_text(encoding="utf-8")
+            self.assertNotIn("build_region_index(emu_profiles", source, module)
+            self.assertNotIn("build_region_index(profiles)", source, module)
+            self.assertIn("platform_region_index(", source, module)
+
+
+if __name__ == "__main__":
+    unittest.main()