1. Select Tools->AutoCorrect Options... 2. Add e.g. degC in the "Replace" field and °C in the "With" field The strings choosen are irrelevant. 3. Select New and OK. Result: The file acor_en-US.dat seems to be updated but the replacement is never stored. If you enter Tools->AutoCorrect Options... again the strings you have entered are gone. The problem is the same in Calc and Writer. I think this worked in LibreOffice 6.2.
I have tested this in Ubuntu 18.04 and Windows 10 and the issue is the same.
Looks like a duplicate of bug 128659.
Indeed it's a dup. *** This bug has been marked as a duplicate of bug 128659 ***