Battery shows lightening icon, but it not charging

Hello,

my old iPhone 11 Pro Max, suddenly did not charge anymore....

I have purchased a new battery, bit still it does not charge... the battery came charged and I can see that when I plug the power cable, the lightning icon in the battery icon is visible, but the battery does not charge... it actually "decreases" the percentage...

it seems that the motherboard "sense" the power cable, but there is somethgin preventing the battery to re-charge...

having already replaced the battery, so I can exclude a battery issue... what next?

what would you try to change/try to assess?

Thank you