Extremely high heating costs with underfloor heating. Incorrect setting?

I find 18-20 degrees quite low. In the evenings here it is now (so October 24 degrees). For that reason, I have set the heating curve higher so that the heating turns on at an outside temperature of 10-15 degrees at all. If you find 18 degrees sufficient, the heating doesn't have to work hard during the day. That might also explain the difference to the other apartment if, in addition, one room has to be heated along. Warmth is very subjective, after all, and for some, 24/7 it's the most important point when it comes to living space and they prefer to air in cold. Others think it's cool to save on heating and prefer to sit with a cardigan in the living room to have saved a euro again at the end of the month.

I have raised the heating curve so that the heating turns on at an outside temperature of 10-15 degrees.

Typical problem of the transition period.
For this, the heating curve is shifted vertically. Depending on the system, this can be controlled differently.
However, the heating curve itself is not changed. You adjust again directly when it gets colder.