I agree with your one resource that says it is just a bug, as MSP does have a few. However, we often find the following workaround helps with many of these”bugs”
I also work in the same default settings you have with fixed duration, no effort driven.
Make certain also that your constraint is set correctly (ASAP for forward pass and ALAP for backward pass schedules). I have found the that constraints unexpectedly change so much that I have inserted that column to keep an eye on them.
Ctrl X your resources, do the changes you need, then Ctrl V your resources back.
Failing that, sometimes you have to just re do adding in the task.