Note: My version is still reported as Beta 1.16.220.50. Not 51.

They now just seem to sit in the water completely ignoring any flow. They still retain any horizontal momentum so do move a bit in water sometimes, just not related to it's flow direction.

This seems to just be a client-side issue. If you make the water stream long enough, you will still get the experience from the orbs and they will start to disappear even though they are nowhere near you.

I think this may be related to MCPE-11181 but it affects all orbs, not just some and I'm pretty sure flowing water affected orbs properly in previous releases where MCPE-11181 was still present, so definitely not exactly the same issue.

The exact issue can be hard to spot is some situations since, if you are close to the orbs at all, their attraction to you will completely take over. The now very few orbs coming out of a water stream trident killer will have been pushed in the direction of the water stream by the last piston before the opening so it looks like they are just having a lot of trouble moving with the water. The ones now stuck in the trident killer that should be carried out by the flowing water are constantly being moved by the pistons so is harder to notice the problem.

I have attached a custom world that illustrates this by drooping both XP orbs and items into the middle of a water stream that eventually carries them to the player (when near all the controls). This makes it very obvious as the XP orbs will float upstream if they started with momentum in that direction. They will all just stop in the flowing water stream where their momentum runs out. The items on the other hand are very quickly accelerated in the direction of the water flow regardless of starting momentum.

If you switch it to survival and place your self at the end of the water stream you will see your experience increasing after enough time for the orbs reach you on the server side. You can also see the orbs start to disappear near where they were dropped at this time.

This makes it quite hard to design farms that generate XP that needs to be carried to the player either because the XP is a core part of the farm or because it will very quickly start to lag the server. E.g. Silentwhisper's Guardian farms or some gold farms.