Hi Thanks for reporting. Unfortunately, for the first, unless you click on the thumbnail being created as you noticed, it seems to continuously update, on the 32-bit version. This does not happen on the 64-bit version. So, it is harder to track down.
The second is due to a change in behavior within Gimp itself. I suspect it was in response to another bug and so may vanish soon.
Hopefully you won't be deterred by these issues since 2.8.1 is still a solid release.
I consider these bugs to be minor and so have not spent time on them. I am working on other stuff.
In the meantime, if you cannot live with these 2 issues, you may have to go back to the 2.8.0 version where you will find the other bugs, or return to 2.6.x.