As you know, the polymorph effects of Polymorph Self are occasionally reinstated with the accompanying shapeshifting effects (green light and roaring) when the spell is still running. I don't really know what the logic here is since this also seems to increase the duration of the spell beyond what it should be, but it appears that when this happens underwater, your speed is bugged until you get out of water: you move as if you were heavily encumbered.

This has been verified to happen several times. I don't know if canceling the polymorph solves the issue, still need to test that.

I should add that I wasn't carrying enough stuff to be encumbered even if my strength wasn't modified due to polymorph.

Confirmed, I've seen it happen on druids that shapechange into "spoiler"

Also, that green light effect causes mage armor to go away. It is a real pain the A**

It causes any buff to dissipate in a short while, it seems, except for the polymorph itself. (Regardless of whether you're underwater or not.) Really weird.

I think it is the same thing as the bug with the wildshape. If it is then it does not fix your speed problem by coming out of it, just makes you more likely to die.

I know that this effect occurs whenever the server saves, wether it is a manual or an automatic save. In the old times, a save caused all polymorph spells to end, no matter their actual duration, but I don't know how it works nowadays.

One thing though is that this makes all of the shark wildshapes extremely useless, or at least unrelyable, when you have to swim along at snail's pace..

Any news to this?

Still bugged, just recently tested again.

Not having any armor or weapons/shields equipped did not help either..