@tpalmer Sorry for the delay in getting back to you.
I checked and I do see Description and Priority being updated.
The output bug remains:
I am able to assign later actions to use dynamic outputs from the Update Work Item, like Description, however when run, the output is missing that content and the later actions just insert empty strings in place.
If the issue is that the VS API doesn't return the values, the solution would be to implement a Visual Studio Get Work Item action to work in conjunction with the Update Work Item action. This is a pretty glaring omission and I believe I've requested this in the Flow Ideas forum already.