Battery gauge goes down but not up, but does charge

The battery level gauge of my Asus Z8 tablet with Android 5 has been behaving abnormally of late.
When I connect the charger, the charge level icon does not show power is connected and the battery level does not move. The Ampere app shows that device is on battery. However, the battery is indeed charging and Ampere shows the battery voltage creeping up. All this time, the battery gauge shows the same level, say 40%. I let it charge for a while. The meter still reads 40%, After a bit of use, presumably when the battery level goes down to 40%, it starts going down normally. I carry on using it, and the level displayed creeps down, to say 20%.
Connect the charger again, the level stays stuck on 20%, but the battery voltage goes up.

A reboot corrects the battery level, but the behaviour stays the same, except at the new reading.
I can get an estimate of the amount of charge from the battery voltage on Ampere., so it is usable, but a pain.
What I am most worried about is that the since the OS does not appear to know the unit is in fact charging, it could overcharge if I were to simply leave it to charge overnight as I usually do.
Any ideas anyone?

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