Can confirm the issue when testing on KAVERI (another Sea Islands GPU)
error <general>: ffmpeg[0xc93ecf0]: [h264] Hardware does not support image size 1920x1088 (constraints: width 64-0 height 64-0).
error <general>: ffmpeg[0xc93ecf0]: [h264] Failed setup for format vaapi: hwaccel initialisation returned error.
Seem to be a kernel 6.6 radeon driver regression, when switching to amdgpu by adding
radeon.cik_support=0 amdgpu.cik_support=1
to the kernel parameters (APPEND line of /flash/syslinux.cfg) acceleration is working fine.
On kernel 6.11 of LE13 HW acceleration is working with both radeon and amdgpu.