The spec finally clarified the meaning of the field to:

> maxLevelIdc is a StdVideoH264LevelIdc value specifying the maximum H.264 level supported by the profile, where enum constant STD_VIDEO_H264_LEVEL_IDC_<major>_<minor> identifies H.264 level <major>.<minor> as defined in section A.3 of the ITU-T H.264 Specification.1

Annoyingly, we have to convert it to an IDC value to compare it.
Mesa commit:
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24649
Only mesa currently uses the IDC value, all other drivers use the enum.

Patch attached.