It sounds like you are facing the same issue as
https://github.com/pagespeed/mod_pagespeed/issues/1048
I have a workaround for you, but I'd really like to know the answer to a few questions.
1. Do you have memcached connected to mod_pagespeed?
2. If so, are you hooked directly to memcached? Or is there some other program speaking memcached that you are connected to?
3. Do you have LoadFromFile configured?
4. If possible, could you send us your entire pagespeed configuration?
I am fairly certain that you have InPlaceResourceOptimization enabled. That was turned on by default in MPS 1.9. If you turn it off and flush cache I think you'll find this problem doesn't show up. This is not very satisfying to us because we believe InPlaceOptimization is an important feature and we want to make it work perfectly.
Please let me know the answer to the 4 questions above, and whether or not adding
ModPagespeedInPlaceResouceOptimization off
to your configuration fixes the problem for you.
Thanks,
-Josh