Could you please provide more details?
I guess that happens when the Item state is UNDEF (e.g. after an openHAB restart when the binding has not not updated the state yet). UNDEF obsiously cannot be parsed to a number, therefore NaN (not a number) is displayed.
I have checked the slider, it seems to command the state when the user interacts in that case following this scheme: Use minimum if defined, else use maximum if defined, else use 0.
I have implemented the same behaviour for the stepper in `oh-stepper`: Fix NaN shown for Item state with unit & Fix unable to control Item with NaN state by florian-h05 · Pull Request #2137 · openhab/openhab-webui · GitHub, however please note that NaN is still displayed if the state is not a number - this is correct.
If you confirm, that this happens when the Item has no number state (i.e. UNDEF or NULL), I will un-draft my PR.