About the only difference plugging new phone in to filter vs old phone in filter, and new phone into the socket filter is plugged into, is the quality of the connection between plug on new phone and socket in the filter.
Many sockets have four parallel strips of copper or some alloy. Depending on how tight a fit, and how long the old phone has been plugged in, those might have become a bit less 'springy' so might make poor contact.
Alternatively, could there have been any moisture or oxidation affecting those strips ?
Not sure of an easy way to 'clean' them but perhaps pushing a really fine screwdriver with the blade flat against a strip (to force off any dust/ greasy /oxidation deposits) might make the contact better. Also check the shininess of the plug - should be nice and clean copper, but if it was in a container and was dampened by sea water, there's a 0.0001% chance the contacts have suffered oxidation, with green or white surface...
Hope that helps. Alternatively, look at Solwise (.co.uk) and for a fiver or so, get a new filter (always handy to have a spare) and see if that works better. Good luck.