That you don't know this function and stumbled over it still does not mean it is a bug.
There are functions to avoid the behaviour that you noticed and there are a lot of threads that deal with multi-value fields and the way to treat them.
e.g. here:
If you want to insist that the documentation does not say it, then see this explanation about "Replace:
Replaces the given Original string from the selected Field with the Replace string. If the replacement string is empty, the original string is removed.
It says "Field" and not "Fields" or "type of field".
Already the failure to replace the \\ should have triggered a warning for you.
If you want to avoid such problems in the future, do not use multi-value fields (which are evaluated by only very few players anyway) but use a single field with a separator that is not interpreted to split up the fields.
The semicolon that you chose in the initial run of the (failing) replace action may be a good approach.
In short: surprising behaviour is not necessarily buggy behaviour.