I use it singe years - see: Problem with charging my phone - #2 by zaphod_42
I don’t know how accurate that Ampere app is. I have USB volt and amp meter, and it was saying 800mA was going into the phone while Ampere was saying 100mA. The USB meter is Keweisi brand. You can get them eBay pretty cheaply.
I think Ampere App is measuring the current into the battery. Negative values mean that the battery is beeing discharged.
Welcome to the world of electrical engineering, it’s a fun place where you can never be sure of anything until you get shocked
All jokes aside: always remember that in electronics your knowledge is only as good as your measurement tools. Note that ampere is software and can therefore only give you the knowledge that the underlying android OS is providing. There might be all kinds of filters, transformations and checks in front of it.
Also note that your “cheap device” might have low-quality transistors and connectors which could skew the result
If you want to be 100% sure you have to get a good multimeter and hold it to the adequate lanes in the USB-head (which is terribly complicated). Otherwise you need to keep switching cables and wall plugs until you have a high degree of confidence of which output gives the most true result