There was a regression with https://z0r.de/6483 (SWF) a few weeks ago.
Yes, it's AVM2, but it used to work well before.

On a912735 the walking robot animation appears to work fine.
Then on the next commit, 5054651 it just disappears after the first frame, outputting lines like:

[2021-03-28T14:15:30Z ERROR ruffle_core::library] Tried to instantiate non-registered character ID 1
[2021-03-28T14:15:30Z ERROR ruffle_core::display_object::movie_clip] Unable to instantiate display node id 1

and:

[2021-03-28T14:15:32Z ERROR ruffle_core::display_object::container] Encountered AVM2 error when dispatching event: TypeError: undefined is not an Object

And on the commit after that, a2d5601, the result is the same, but the second kind of error is not printed anymore.
This is also what happens on current master.

These commits were merged as part of #3177.