Well, it could be a “bug” in that it was an unintended side-effect of some other changes, but it could also be intended. I’m not sure where you heard or read that belt rats were not suppose to enter mission sites. Just because they never did before doesn’t mean CCP changed something as part of their work on the economy and they are suppose to now. Or maybe the bug was that they always were intended to spawn wherever mining was possible but never did for 15+ years and someone fixed that. Or maybe, when they removed most mine-able rocks in missions, they changed it so that the normal belt-spawn mechanics were now active for the few they left.
In any case, if the economy/ecology team is looking at this, I bet they would be happy with this even if it was unintended. I’d also be curious if CCP has given you or someone else a response on this apparent change.