I just discovered it works for me too, if I use the export preset I made before, but if one starts fresh from lr export presets, the tick box Use Lossy Compression doesnt seem to respond. Its probably just gui bug?
I am also able to reproduce this issue. Once I click on an Export Preset, I am not able to check "Use Lossy Compression". We will file a bug and investigate it further.
Updates to Lightroom Classic, Camera Raw, and the Lightroom Ecosystem were released yesterday and contain a fix for this issue. Please install the appropriate update and see if it solves this problem. You can read more about the updates here.