LrC 15.6 (macOS 27.2): H.264 video import and playback hang, dynamiclinkmediaserver stuck at 100% CPU in mc_dec_avc

Description of issue:

On macOS, every H.264 .MOV file fails to import ("There is a problem with the video file") or the import stalls, and H.264 videos already in the catalog will not play. The dynamiclinkmediaserver helper process spins at 100% CPU indefinitely, and a new stuck instance is left behind after each Lightroom launch. Simply opening the Import dialog on a folder containing an H.264 clip is enough to trigger it (thumbnail generation). HEVC versions of the same clips import normally. Disabling "Use Graphics Processor" does not help.

Lightroom Classic version: 15.6

OS version: macOS 27.2 (26B123), Apple silicon

Camera model: iPhone 16 Pro (iOS 26.6.1). Also reproduces with a plain H.264 file re-encoded by Apple's avconvert, so it is not specific to the camera.

Steps to reproduce:

1. Put any H.264 .MOV (e.g. iPhone footage exported from Photos: 1920x1080 30 fps H.264 + AAC) in a local folder.

2. Open File > Import and select that folder (just loading the thumbnail grid is enough).

3. Watch dynamiclinkmediaserver in Activity Monitor; optionally start the import.

Expected result:

The clip appears in the import grid and imports, and catalog videos play.

Actual result:

dynamiclinkmediaserver goes to 100% CPU and never recovers. The import stalls or fails with "There is a problem with the video file". A process sample shows all time inside the MainConcept AVC decoder:

ImporterMPEG::OnOpenFile8 > MPEGInstance::AllocateMPEGInfo > mpegInOpen (mc_mfimport) > sessionDefault (mc_dec_avc 14.3) > [mc_dec_avc +0x2885b4 / +0x25ace4] > free (libsystem_malloc)

lsof shows the helper holding the H.264 file open. The media cache shows ImporterMPEG returning -1610153856 for earlier clips before falling back to a second importer; it now hangs instead of failing over.

Ruled out: damaged file (decodes fine with AVFoundation), network storage (fails locally too), file attributes/permissions, Live Photo metadata tracks (removed, still fails), GPU acceleration. The app bundle and the decoder framework pass codesign verification.

Possibly related (Windows, mostly HEVC, error instead of hang): https://community.adobe.com/bug-reports-674/p-lrc-15-5-video-files-issues-windows-1635342

Same issue already reported by another user (macOS 27.0.1, Apple M6, mc_dec_avc 13.2, also failing on 15.3.1 and 15.4.x), currently in Questions: https://community.adobe.com/questions-675/h-264-video-import-consistently-fails-with-dynamic-link-server-error-on-apple-silicon-mac-1645043

This report is an independent reproduction on a second machine with a different decoder build (mc_dec_avc 14.3).