There are quite a number of threads about this issue already. Something is awry in AE version 13.6, but it doesn't affect everybody.
Todd Kopriva from Adobe has posted about it in several similar threads:
regarding the issues of image information being incorrect after Undo or delete or similar operations:
We are very keen to isolate the causes of these issues. We have a couple of engineers visiting someone having this problem...so that they can debug the issue.
Please, submit detailed bug reports here:
If you provide your contact information, we'll be reaching out to some of you.
The issue is that the information held by the render thread is getting out of synch with the information held by the UI thread. This kind of problem can be worked around by either restarting the application or purging the caches---either of which forces a re-synchronization of these sets of information.
It is guaranteed to work if you purge your cache (because that resets both threads and makes them behave), but that's frustrating to do all the time.
You can roll back to 13.5, install 13.2 (CC 2014), or learn the keyboard shortcut for purging...
A few people have found fixes that worked for them:
Possible fix from: https://forums.adobe.com/message/8245129#8245129 - right click the AE icon and choose to run AE as an administrator
Possible fix from: https://forums.adobe.com/message/8251043#8251043 - reinstalling mouse driver
Possible fix from: https://forums.adobe.com/message/8246053#8246053 - reinstalling AE and its update
So try those.
If you'd like to help us to test the After Effects CC 2015 (13.6.1) update---which fixes this bug---you can request access to the prerelease testing program here:
Choose the "Private programs" tab and click Apply to apply for the "After Effects CC 2015 (13.6.1) patch update" program.
I can confirm the update solved the problem on my windows 10 machine.
Thanks for confirming.
I still have version 18.104.22.168, and no updates are available in my Creative Cloud app at this moment. Do they take a while to show up?
I'm having lots of problems with the undos and refreshes, to the point where I can't properly work on a project.
Sometimes, choosing the Help>Update option in Bridge works and sometimes logging off, closing, reopening, and logging back in to your Creative Cloud account works too.
But sometimes it's just not there yet because it can take a while to propagate around the globe.