Hamza1639
Depends on the style of seal.
My guess one or more of the following will be happening,
1)When the pump is running it is actually operating at negative in the seal area and is pulling air in through the seal so that little if any water leaks out.
2)The spring that pushes the faces together has lost its tension or perhaps the O Ring shaft seal is damaged or fouling on the shaft such that when there is no pressure behind it(when the pump is stopped) the seal weeps because the two faces are pushed together.
3) The housing or shaft is damaged and allows water past it when it stops but is under sufficient negative pressure when running that the leakage is minimal.
4) When stopped the pump is pressurised in some way(perhaps by another system or faulty auto priming system) that is beyond the capability of the seal to maintain when stationary.
5) The pump cools down when not pumping the hot water and the tolerances or tensions change as it cools.
It may be that the seal also leaks when the pump is running but for various reasons the drops are much harder to find or see.
Almost certainly the problem will require a pump dismantle and a seal inspection
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"