Did you find anything?
I recently upgraded many controllers from 5.0.3.3 to 6.1.3.6 and am starting to get the same complaints.
We use AP-93, with single 2.4ghz radio.
I'm fairly certain the issues are with actual coverage in our offices, but the complaints only appeared after the upgrade. In one extreme case I rolled the controller back to 5.0.3.3. I called the next day to check and was told the issue seemed to resolve itself.
I'm in the process of adding APs for coverage, but that doesn't help the daily calls about the suddenly reduced signal and frequent disconnects.
I actually need to be at some version of 6.x code because the new APs are not compatible with 5.0.3.3. My SE recommended 6.1.3.6 with the caveat to check release notes for any known issues......I didn't see anything in the release notes that would affect our network.