Upgrades still canceling unprompted?

I think I've accidentally identified another circumstance in which upgrading units will cancel their upgrade despite not being directed by the user.

I had a unit upgrading on a province owned by my enemy, while another unit headed towards the province center. I wasn't watching when it happened, so I can't confirm with 100% confidence, but I believe the act of capturing the province automatically canceled my other unit's upgrade. Thankfully it was just one unit but hopefully this is something the devs can look into.

Edit: after looking into it further, I'm not very confident that it happened this way vs me just forgetting to upgrade a particular unit. Ignore for now.