After updating my realm and client to software version 1.17.30, I have 3 instances of mobs being released from vehicles.

After first going back into my realm after update, A tamed wolf teleported to me.  It had previously been in a nearby room trapped in a boat.  Next, a Vex which was name tagged and in a boat was found flying over where it had previously been on display.  Third, a horse was placed in a minecart and sent on a thousand block ride.  I flew along with the horse but after doubling back and then returning the the chunk where the horse was, I found the horse was no longer in the minecart.

These 3 instances all happened within 10 minutes of each other.

This bug is very serious.  It represents the loss of several hours to collect rare mobs and makes minecarts essentially untrustable and unfun!!