This is not something that should happen and I cannot reproduce it.
I suspect that this is being caused by an add-on or some sort of custom caching mechanism (there are a few cache add-ons available).
Ultimately, you will need to disable all add-ons and start enabling each one until you find the culprit. Bear in mind that just disabling the problematic add-on won't correct the last post info, that would only be updated after the change there is triggered.