I can't enter passwords in Safari 11.1 on macOS 10.13 High Sierra because the last character gets removed
I assumed you checked your accessibility and keyboard preferences. I have had numerous issues with keyboard setting being modified/added during the updates. sticky keys and other settings were modified that I wasn't aware of...I had to start using the manual keyboard viewer until I was able to get it fixed...

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only other thingi I can think of is trying on an alternate or guest login to see if you have the same problem on a clean user.

I know I am still struggling with my fare share of HSierra simple glitches but I hope you can get this one solved! Good luck!