Under some circumstances, the SourceBuffer.append() can generate an EOS internally in glib ports, which reaches HTMLMediaElement as DecodeError. This triggers detachMediaSource() while the player is still playing fine (it's not aware of any issue). Eventually, the player reaches EOS when running out of samples and the playback is ended. The JavaScript app may try to resume video with video.play(), triggering seek(0), which triggers other internal operations. One of those operations was a MediaPlayerPrivateGStreamerMSE::sourceSetup() call that was trying to setPrivateAndOpen() on the MediaSource that was already detached and this was crashing. The crash itself was fixed in https://bugs.webkit.org/show_bug.cgi?id=220091 by reopening MediaSource on load() only, but the player private still keeps this detached MediaSource that may cause different issues.
    
The proper way to avoid this invalid state where a reference to MediaSource is kept by the player private is to ensure that the player is destroyed on error. It will be recreated again if needed if a fresh src is set in the future.