I’m not exactly following what more details I can provide. I’m guessing you are saying that NaN would be correct right after a restart. Agree. However, in the past the actual value of the item was populated shortly thereafter from the restore on startup persistence. In my case the Items tab shows the restored value (number:temperature), but the stepper still shows NaN. My other observation is that it seems to be locked as NaN. I could not get it to go up or down without the Communication failure message noted by @DrRSatzteil.
As noted above the slider worked, although I did not test how it handles a restart, although I haven’t seen a post about sliders not working after a restart.