Probably the same issue reported here, and here.
Something has changed (probably in the kernel) that has subtly broken udisks detection of disks. (Nothing was changed in our udisks configuration in the time between when it was working properly and when users started to report this problem)
Still trying to figure out the exact cause, which is not helped by the fact that I cannot reproduce the problem at all on my own hardware. (Auto mounting works 100% for all the drives I’ve tried on my Pi 2)
By the way a full log is helpful when trying to diagnose these problems - log snippets don’t really help because you don’t know what we’re looking for when we’re trying to diagnose the problem.
So please try to reproduce the problem then upload the full journal with:
sudo journalctl | paste-log