Battery charge level not changing after replacement.

I decided to replace the battery in my Lenovo android tablet. I obtained one through Amazon and carefully installed it. It seemed to work fine, but the charge level never seemed to change from 57% if it was in use or on charge. Eventually it just died and I could not get any response from my tablet (even on charge).

I replaced the battery with my original one and that is now behaving in the same way - but permanently showing 50%.

Yes it was working fine before (though needed recharging every couple of hours).

And yes I have done a factory reset- still saying 50%

I have cleaned and carefully checked the battery contacts, and ensured the connector is fully engaged.

Any suggestions?

Most googling comes up suggesting the micro USB port is faulty - but this would not explain why the level indicator does not go down from 50% when in use!