sorry, I use Teams only on work devices - Whatsapp so, does what you describe - if I leave the WA window open in the back, the last WA chat open before bringing another app window to the front to e.g., type in Archimedes is indeed updated (as well as you get the normal notifications). Can’t really say whether that is synchron with WA on my iPhone so, or whether there is some short delay.
Another thing to test is sending an active video chat in the background and see what happens - will do that one later.
WA and Teams are currently often updated it seems - so there is that… might work in Teams too 😃
Anyone else can contribute?