With the purple box active, or having sized it, before clicking, press TAB to “lock” it, then use ‘T’ to alter the thickness. It just may be the default thickness is super-thin. Works fine on my 3.2.1
The oddity with the flickering is multiple inset booleans getting conflicts, just shifting the inset slightly corrects it. That’s just a boolean gotcha really, which is why sometimes people apply so many booleans before doing others.