I upgraded our MR45 AP's (roughly 200 of them) a few nights ago. I started fielding support calls from our buildings right away the next morning. I currently have a case open with Meraki to try to find out why. The second we roll an AP back to 26.8.1, the issues are resolved.
Our users were seeing low signal strength even though they were connected to the AP in their respective room. Some of the AP's showed multiple reboots on Meraki's side. I wish I could give you more details but I have a few other fires that have a higher priority at this time. I will send an update in the next day or two once I have time to do some more testing.
My temporary fix was rolling all AP's back to 26.8.1. Everything is running fine again but I can say for certain, 27.5 seems to be problematic on MR45 AP's in our environment.