So it's clearly a bug or intentional misfeature by Apple.

That sucks, if it is the case. It only takes a second or two for a newly added number to switch from green to blue (I'm sure it's some kind of HTTP number lookup request to an *.apple.com web service), so why couldn't they re-request that every time - or at least keep the caches down to like 15-30 minutes...

Do iOS apps not have a clear data/clear cache system level option like android does?

If not, what do you do to wipe an app's data?

It does. Since it's cached on every device of the people that has sent messages to him, each one of them will either

1. Wipe out the iMessage data - which is all their message history.

or

2. Manually delete every message/thread the OP has been involved in.

That's not going to happen.

I believe you're missing ac29's point. Android allows applications to distinguish between cached data and "regular/permanent" data. It also allows users to clear them separately.

Logically, the Apple ID / phone number mapping seems like it should fall under cached data (e.g. it can be regenerated and does need to be re-checked from time to time). If the iOS Messaging app properly implemented this distinction, users (or Apple) could clear this cached mapping without touching anyone's message history. Sadly, it sounds like it hasn't been implemented that way, so...