Experience is not being awarded to commanders that have ascension levels available. Just another bug to add to the list of unfortunate events that the patch has incurred. Experience works for the core levels of a commander so you can still earn Mengsk level 15 at least. I hope players get a refund of the lost ascension experience they are missing out while this is ongoing.

EDIT: For clarification. The post lobby shows the experience earned. However if you check the mastery banner button on any of your commanders, the bar to earn another ascension level will not be shown any progress. It will be frozen at the current amount of experience you had before the patch.