Not sure I can be much help here. There is certainly a known (I wouldn’t say “common”) issue of water leakage into the rear footwell; videos on YouTube about this. The usual cause is detachment of the door’s internal plastic membrane, but you say this has been checked so there must be something else going on. Check the drain holes underneath the door are not blocked. If the rubber seal itself is deteriorating then that ought to be detectable by inspecting it, looking for perishing, holes, or some foreign body obstruction (including underneath the seal). I think you can probably check whether it’s still sealing against the door by chalking round the seal.