@CmdrMic Hi thank you for letting us know. I don’t have an up to date linux mint distro to test, but may I inconvenience you to test if this happens with the flatpack version? The instructions are on the download website but you can also take a look at flathub for more info
https://www.flathub.org/apps/details/org.pencil2d.Pencil2D
Now, since XPlayer does run the video properly when running from the folder itself, there may be some kind of flag or command that needs adjustment for the Linux conditional code in Pencil2D. But that’s just my intuition, I personally wouldn’t know where to look for this 
Let’s wait for a developer to take notice of this issue as well so they can participate in the discussion.