You are right, it is not the size!
Before reporting it as a problem, I had to check it in more detail and see if it was possible to recreate it.
It turned out to be users in /etc/passwd with names containing \xE5.
This is small a with a ring above, but in Unicode, CP1252, 8859-1.
Most likely the users are imported from other systems with other codepages, and there are more than 2500 of them!
It is very easy to recreate this problem. I have done this in two other installations, RedHat 7.9 and 8.5, BigFix 10.0.5 and 10.0.9. At least RH8.5 is a plain installation with no extra security tweakings.
USERNAME=$(echo -e -n “P\xe5l Paljett”)
useradd -c “$USERNAME” -s /bin/bash -m paljett
Then create an analyse of the Upgrade fixlet, and check the result.