Suddenly I notice that the effect of home made Feather potions won't stack. If gulping down three bottles, the first works as it should - under Active Effects I get Feather - Potion: Feather (72). But for the others I get Feather - Potion: Feather (0). I tried out other restore and fortify potions and their effects stack so seems to be only Feather that is affected. Pretty sure it's a rather new issue as I use Feather potions now and then - but I haven't added any mods that should affect it as far as I know. Not sure how to check it in TES4Edit either. So - anyone who has an idea?

How much are you carrying when you're gulping down the feather potions? The Encumbrance display in Oblivion is incorrect, and leads you to think that feather adds to the amount you can carry, while in reality it removes weight from the things you already carry. So if the things you carry weight 72, there's no more weight to apply the additional feather effects on. So wait drinking feather potions until you have more things to carry - and if you want a correct display of your encumbrance when using feather effects, install my HUD Status Bar mod

And the interesting thing is that they did this properly in Morrowind.

What happened? Did they just space out on the UI?

Anyways, yes, do check that you are not trying to feather more than you are currently carrying, for the reasons listed above.

That should be it. Normally she takes Feather potions when encumbered - now she took it preemptively as she was about to check out a bunch of Daedric armor replacers. I have read about the odd way it is implemented but hadn't experienced this effect of it before. Thanks!

EDIT: Yes, that was it. I haven't considered HUD Status mod to be honest as I want to have as little as possible on the HUD and thought it mainly was about adding/integrating new HUD showings, but that seems like a neat feature. I'll check it out.