This is a very old design feature. You only have a short grace period of 5 minutes for undoing a vote. AFAIK, this is a network-wide feature, I haven't heard of per-site exceptions.
What are the limits on how I can cast, change, and retract votes?
With very little searching, I found a reference to it from 2010: Unvote (toggle vote up).
It is possible that it is older than that, maybe even from the beginning of SE.
I found some old discussions of the change. It was done by the network founder Jeff Atwood and must have been in place in February 2010 at the latest. Why can't I change my vote if the post has been edited during the initial 5mn grace period?. He refers to it even earlier, in August 2009: https://meta.stackexchange.com/a/18046/149055. It seems that it was unpopular back then, but it appears to be considered status-by-design.
The rationale behind it is prevention of certain type of voting "fraud". There are examples discussed on Meta Stackexchange. As to why you haven't noticed it before, you might have always undone a vote within the grace period before now.