I am still seeing this behavior on new machines even after upgrading to 10.15.5. I set up 3 new MacBook Pro's (2 x 16" and one 13") and out of the box we installed an extension needed for our district. Then applied all updates (security, safari, and system) and the same issue resulted. Could not click the check box. Could not resolve by moving the preference pane around the screen or to the middle. Only resolve is booting into safe mode to enable it.
With some of the WWDC info it looks like extensions are going to be completely redone for Big Sur, but until then we really need this to be looked at.
GabeShack
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Princeton, NJ, United States