uneven build plate temperature?

99% of my Mini2 printing has been within the centeral region of the build plate.
Starting to print a set of calibration cubes, (1 at each corner, centered at + or - 60 xy)
the bottom face peals up warping the print. Is this the result of uneven build plate temp at the extremities? What to do? Common problem? Unexpected feature?

What type of filament are you using?
Do you have an enclosure on the printer?

Thanks — PLA Polymaker Lite and Matte Forge PLA.
No enclosure… For PLA?

Finally, I have printed a set of calibration tiles in Lulzbot Green Polylite PLA, .25 nozzle and .05mm layers. Still a warping problem in the back right area:


…and 2nd image