This thread might be out of date but says that there’s 3 total kanji which this is the case for, and both the others are for more actual names they’re used in rather than compound readings. So it seems like the exception
I think technically, like someone said in that thread, it’s not even a nanori reading, instead it’s a 熟字訓 more like 今日 as you said. So it’s probably better to think of it as a special pronunciation.
You’ll see 叔父 and 叔母 quite frequently, so it’s fine to just learn it in those words, and not worry too much about it like Vanilla said