I tried with both the probe measuring ambient and beer temp and had the same problem. That freezer would simply cool too fast. It might only drop my beer temp a dregree before the fermwrap kicked on, but that was still playing roller coaster with my yeast. I now use Ranco dual stage controller AND a second Johnson controller. For an ale attempting to ferment at 67 degrees I will set both the heating and cooling on the Ranco to 67 with a 1 degree differential. The freezer will cycle on at 68 and the fermwrap at 66. Probe for the Ranco goes into a thermowell in the beer.
The freezer cord plugs into the Johnson which is set to shut off at 60 or below, probe measures freezer ambient temp. The Johnson gets plugged into Ranco cooling plug. I can adjust the maximum cold temp of the Johnson if necessary, but so far 60 seems to be a good number. This all keeps my beer dialed in without any swings.
By the next weekend, the temperature rise problem is typically done which will free up the chest freezer. My basement temps are 64 in the winter, 67 or so in the summer. I keep the fermwrap on throughout fermentation and can use another Johnson to control it if doing consecutive brewing weekends. Just need to switch probes in the thermowell.