Hi Anna,
I don’t know enough about this to answer confidently, but from my read of the github issue I shared above, I suspect this is caused by the same issue indeed. Some video codecs such as VP8 or VP9 support a variable video frame rate, and your webm videos were probably encoded using those.
Here is a reddit thread that gives pointers on how to normalize variable-framerate .webm files. You may need to look up some ffmpeg options if you are not familiar with them, as not all are explained in the thread. Also, feel free to use the specific keywords vfr (variable frame rate) and cfr (constant frame rate) in your searches if you look for more information about this process
Jeremy