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?
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.