I created a custom fact type in RM with the definition:-
Name: Birth Alternative
Abbreviation: Alt. Birth
If you upload a new fact to Ancestry. It appears on the website as "Alt. Birth"
Now edit he text of the fact either Place, Place Details or Description then resysnc with Ancestry. It changes on the website to "Birth Alternative"
Now make a dummy edit and save so you can compare a 3rd time and the facts don't line up.
I have seen this before also with the fact "Census (family)" that has abbreviation "Census (fam)".
i can work around for custom facts by keeping the name the same but not for the built in facts as these are greyed out.
It seems the update routine and new fact upload use different fields for the work when sending to Ancestry.