Just tested this on meet.jit.si this morning
When there are only two users in the meeting, when user A shares their screen, user B sees it just fine. However, if user A then stops sharing their screen and starts sharing it again, user B does not see it (nothing appears for user B). At that point, if user B rejoins, he sees the screen share.
We were able to reproduce this consistently (or at least 90% of the time), tried with different browsers (both Brave and Edge).
If there are three users in the meeting, the above does not happen (screen share seems to work correctly), so I imagine this is related to P2P.