A bit more info that may highlight what the problem is:
Playing the flac stream using ffplay in debug mode shows the following at every track change (highlighted by all caps note):
[AVFilterGraph @ 0000015ea37c6b80] query_formats: 2 queried, 3 merged, 0 already done, 0 delayed
[flac @ 0000015ea3695040] skipping flac header packet 1 0B
[flac @ 0000015ea3764980] skipping vorbis comment sq= 0B NOTE: TRACK CHANGE
[flac @ 0000015ea37b9740] skipping flac header packet 1 0B
[flac @ 0000015ea3737c00] skipping vorbis comment sq= 0B NOTE: TRACK CHANGE
In comparison the aac stream played in ffplay debug mode shows:
[AVFilterGraph @ 000001ccf4473940] query_formats: 2 queried, 0 merged, 3 already done, 0 delayed
[auto_aresample_0 @ 000001ccf4470680] [SWR @ 000001ccf449efc0] Using fltp internally between filters
[auto_aresample_0 @ 000001ccf4470680] ch:2 chl:stereo fmt:fltp r:44100Hz -> ch:2 chl:stereo fmt:s16 r:44100Hz
Created 640x480 texture with SDL_PIXELFORMAT_ARGB8888.
Transform tree:
rdft_r2c_float_c - type: rdft_float, len: 1024, factors[2]: [4, any], flags: [unaligned, inplace, out_of_place, fwd_only]
fft_sr_float_avx2 - type: fft_float, len: 512, factor: 2, flags: [aligned, out_of_place]
[https @ 000001cce97ef100] Metadata update for StreamTitle: Colin Hay - Oh California NOTE: TRACK CHANGE
[https @ 000001cce97ef100] Metadata update for StreamTitle: Taj Mahal - Here In The Dark NOTE: TRACK CHANGE
I think the flac header and vorbis comments that ffplay skips to continue playing the stream may be causing the hang and buffer loop in MC33.
I've attached the full ffplay debug outputs for the flac and aac streams as text files FYI.