I am coming across loads of these which have been painted over, maybe they will still operate once the paint melts etc but will they be as effective, should they be replaced.

SBH

At this time there is no evidence that overpainting of intumescent seals has any detrimental effect on performance. There is evidence that there are some benefits in overpainting the seals as they are
less likely to absorb atmospheric moisture. However, there are limits on how much paint can be applied without there being a risk of the seal being rendered inoperative. It is recommended that overpainting be
limited to a maximum of five coats of conventional oil-bound paint or varnish.

Two good answers above, both acceptable when considering fire risk management. BUT, do you know how many coats of paint are on them. Can you ensure that no more than 5 coats will EVER be applied.

What about damage done during the preparation work on the door, ie from heat guns and sanding. Both "can" have a detremental effect on the seal, but paint covers a mulitude of sins so you may not know.

There are too many ifs and buts, so I always advise to replace them if painted over. At the end of the day, there's only one way to find out. Have a fire and see. If you have less than 5 coats or you meet the special circumstances, maybe you will be OK. If not...............