Event duration while playing back is very long

[pcglue]

I am running Zoneminder 1.32.3 on Ubuntu 18.04, installed via iconnor's zoneminder-1.32 PPA. I have events with durations that make sense in the console (~10-30 seconds), but when I go to play them back, they become very long (1,000+ seconds, and I've seen durations of 20,000+ seconds) and don't actually play back. It'll show maybe the first frame and then stop, or it may play a few frames and then freeze. And all the while, the playback counter is counting up.

https://imgur.com/a/DpTozrw

This happens on more than half the events. Other events will play back normally. I have 4 cameras and this happens on all of them, not just one of them. When I play the mp4 file directly on VLC, I also see the long duration time and nothing plays back (it's blank).

Anyone know what might be going on and how to debug/fix this?
Thanks for your help/suggestions.

[snake]

Check the logs.

Does JPEG frame saving work? the library mp4v2 is being phased out, so this has broken H264 encoding on Debian 9, though there is a guide in the wiki for it on Ubuntu 19.04