I have a client that is clearing activities which creates the history entry just fine, but the activity won't clear out of the Activities TAB. The filter to show cleared activities is unchecked. (Also I've not found any help in the other posts.)
The other interesting element is this just started happening two days ago and has affected every user in the network. There are about 20 users. The only change to the environment anyone can recollect has been to install ACT! on a Surface Pro Tablet, which the problem is on that device as well.