It may be unexpected, but it's not a bug. From the documentation:

"""Return a copy of the string with trailing characters removed. The chars argument is a string specifying the set of characters to be removed. If omitted or None, the chars argument defaults to removing whitespace. **The chars argument is not a suffix; rather, all combinations of its values are stripped**.""" (emphasis mine)

https://docs.python.org/3/library/stdtypes.html#str.rstrip

(if you wonder why that is, think about e.g. the use case of stripping certain whitespace characters: s.rstrip(" \t"))