I’ve noticed this lately in several vassal modules (for example Down in Flames 2.0).
I “play a card” (Send to → some Card Deck) which goes to a different window. I have that window open, floating over the main map. But I cannot see the card, it just disappeared from my hand and it’s gone. Until I click on the floating window where the card it’s supposed to be. Poof! The moment I click that window, the card appears. Probably it was there all the time, but I can’t understand the trick.
Are you playing on Linux, by any chance? I use Linux (Kubuntu 20.04), and I’ve noticed that windows usuallly don’t get refreshed when they’re not active. Hasn’t annoyed me enough to file a bug report about it, yet…
When one player plays a card from his hand (after the first one, which seems to work fine) the “Play Cards Board” window won’t automatically update for the second player.