Steps to replicate the issue (include links if applicable):

• Block a vandal account with autoblock and 'hide user' set.
• Note that there is no indication, in any interface, that their IP address is already blocked or autoblocked.
• Attempt to block the (presumably) autoblocked IP address, for example to block it separately for vandalism or to override the autoblock.

What happens?:
The block of the IP fails with this message: "The user you are trying to block has already been blocked and hidden. Since you do not have the hideuser right, you cannot see or edit the user's block."

What should have happened instead?:
I believe the failure should have been silent and the new IP block should have been applied.

I previously reported this in T287798, which was mainly a different problem, and from my experience then, I think it also applies to globally locked/hidden accounts where the IP address is locally autoblocked.

I should probably note that I'm a checkuser without 'hide user' rights, so I can look up the IP. Otherwise I can't really test this stuff. I have no idea if someone with 'hide user' can actually block the IP address. However normal admins will also want to block IPs like this, and without knowing there is an autoblock (it usually doesn't take a genius to link an account with such an IP).

Software version (skip for WMF-hosted wikis like Wikipedia):
This is on today's production enwiki, 1.40.0-wmf.1 (2fc21db), It's probably existed since before v1.37, per the aforementioned bug report.