Shot in the dark, but has your actual stove changed? When have you last changed the stones? Is the circulation of air worse?

If your skin feels hot my guess would be that the steaming effect might be disrupted by the water getting evaporated faster than before, and the circulation of air also affects the skin feel (that’s why a certain seating position can make sauna unbearable). You could also try to just turn it on at the lowest setting and see if it changes anything. Maybe the stones have gotten so old that old heat settings have sneakily turned unbearable.