Video webm transparency - ffmpeg - Settings for importing a video with an alpha channel

I'm struggling to import a video into 2018.3 that contains an alpha channel. I'm getting a "Video Decoding Error" about "defective frames" and was wondering if someone knows the correct way to convert a collection of png images to a webm.

I'm currently convert the images using:
ffmpeg -framerate 25 -vcodec vp8 -pix_fmt yuv420p -metadata:s:v:0 alpha_mode="1" -i "image %02d.png" out.webm