With VDM 2018 SP1 this issue occurred even with a single video.
It seems like the codec gets unloaded before all of its instances are de-initialized.
If you set codec log level to Warning or higher it will display a log window for each codec instance. After closing all videos one of these windows also closes, but the others don't. If you try to switch to these windows LabVIEW will crash as well.
With VDM 2018 SP1 all the log windows stay open, so none of the codec instances are de-initialized.
The codec is the latest version. There are no other reports of codec crashes with 0xC0000005 code. There were no codec or Windows updates. The only update was VDM 2018 SP1 to VDM 2019 SP1.
Considering the behavior changed it definitely has something to do with codec handling in VDM.