Encoding animated avif with alpha has different default behavior based on input

[FrankGalligan]

If you encode an animated alpha piped through stdin the alpha defaults to non-lossless.
E.g.
ffmpeg -i alpha_video.gif -strict -1 -pix_fmt yuva444p -f yuv4mpegpipe - | avifenc --stdin alpha_video_pipe.avif

If you encode an animated alpha using a file or multiple files as input the alpha defaults to lossless.
E.g.
avifenc alpha_video.y4m alpha_video_y4m.avif
avifenc images_*.png alpha_video_pngs.avif