It is irritating to the reader that "caption" is a keyword used for strings, but in CProgressBar "caption" refers to the numeric progress value, i.e. throws an error if receiving a string.
This also stands in contrast to the IGUIObject inheritance pattern where the same GUI setting type is present in GUI objects of different types yet having the same logic (enabled, hidden, size, ...).
We can find evidence even in this file that I'm not the only one:
// TODO Gee: (2004-09-01) Is this really updated each time it should?
The comment was copied from another place where "caption" was GUIM_SETTINGS_UPDATED in rP1232 / rP1237 / rP2116. Despite the later commit, it seems like a copy since it should be quite obvious from the code that it's updated correctly, but not so obvious for the CText caption (also the TODO dates back to a month prior to the commit date).
So the answer to the question is "yes", it is and it always has been (since it only depends on the cached size and the progress value and those have never been copies of states or dependent states that could have become out of sync unlike CInput).