Tested using FFMpeg Release/6.1, Release/7.0 and Release/7.1 and libplacebo 5788a82. Release/6.0 is no longer compatible with the recent libplacebo.

A hardware agnostic issue, tested using Intel DG2 Arc and NVIDIA GeForce RTX 4090 the output is identical on both.

Command example: "ffmpeg -i Alpha_Sample.png -vf libplacebo=downscaler=ewa_lanczos:w=iw/2:h=ih/2 Alpha_Lost.png"

Debug output attached.
placebo_alpha_debug.txt