I dont have this problem on the Matt that is hooked to my Herpstats but I do have this problem for the one on a dimmer I use for quarantine. I couldnt figure out why once I get it steady that a ambient temp change effects the temp of my Heat Mat itself.... Then thinking about how a Dimmer works I think I found my problem... The power used (mines just over 1/2) on my dimmer struggles if the air drops (which it does in NY from 40 during the day to 15 at night) and the 1/2 power doesnt have enough juice to keep it up 91 during a drastic temp drop.
I don't see the percentage power your tstat is running on?