Only charging to 12-19%

Hello

I have recently repaired a customers iPhone 5S by installing a new charge port flex due to you having to wiggle the charge cable to get it to work.

I fully charged the device to 100% battery as did the customer successfully for the next couple of days after the repair.

Over the last few days the customer has stated that the phone will only now charge between 12-19% and then stop. Apparently the charge symbol is displayed on the screen

Has anyone any ideas as to what might be causing this or has anyone else suffered a similar problem? The customer is returning the phone next week so I would like a few ideas to try.

Many thanks in advance