[RandomLensman]

The real issue here is that people stick to the word "Chancellor", not the "acting" as such.

There is no named role of "Acting Chancellor" created in the basic law, instead someone is just tasked with performing certain duties (and people might sometimes call it "acting Chancellor" or whatever). However, the position "Chancellor" is a well defined & special role (unlike perhaps PM) and whatever that other "acting" role is, it isn't a Chancellor. It is a bit like adding Oliver Cromwell to list of English Kings or Kamala Harris to the list of US Presidents - you can do it but you then move beyond the "standard" definitions.

[OJFord]

Charles III was for years described as a 'King in waiting', nobody was confused that perhaps he was already king just because the phrase used that word.

I don't think in English usage there is any meaning attached to 'acting X' which 'caretaker X' (as you're happy to call it) doesn't also carry. Both are used interchangeably, the former you'd put on your CV, the latter might be used by the media when your employer put out the less release announcing it, but same thing: for some reason there is not currently an X, but you are fulfilling some necessary duties that that person would do in the meantime.

[RandomLensman]

When used in a narrow sense "acting X" can capture it, but then sometimes people start to use it somewhat interchangeably with "interim X" etc. and it depends on the specifics what happens in successions etc.