Wrong seeking to a key unit in certain (encoded by Adobe products) ProRes movies (macOS x86_64 & arm64, Windows x86_64, …)

Reproduce: (for macOS)

1. Build the attached do.cpp source file by clang++ with GStreamer v1.20.5 as noted in the file's top comment (nothing special, just substitute the real path to the GStreamer.framework — the output binary will be ./a.out).
2. Put the bridge_timecode_framenumbers_25fps_ME-ProRes422.mov video file to the current directory.
3. Launch the built program as follows: ./a.out bridge_timecode_framenumbers_25fps_ME-ProRes422.mov ; echo $? — it will output how many frames a seek to the nearest key unit spanned.

Expected: the output is 1 as ProRes is an I-frame-only format.

Actual: the output is 25 which corresponds to a 1-second GOP.

Note 1: the issue reproduces both in Windows (within our software, causing skipping/duplicating some frames) and on macOS: on x86_64 (tested in macOS 11.7.2) and on arm64 (tested on M1 Mini 2020 in macOS 12.6.2).

Note 2: the issue is not new to v1.20, it reproduces with our custom build of v1.18.5 in Windows (with AVFoundation-based ProRes support added to vtenc by Nirbheek Chauhan).