So I have the strangest problem. I have 2 a70 phones. One (phone a) sometimes won't charge. The chargers are authentic (came with phones) and work with phone b every time.
I found out by accident once that if I remove the sim of non-charging phone a (as I was going to switch to a different phone) but then put it back in and plug into the charger, it suddanly charges no problem! Wtf!!
This happens about every 6 weeks with the same **bleep** phone and I'm about to lose my mind.
When it starts charging is the SIM tray out also? It sounds like there is a short.
You should also try cleaning the charging port a lot of times lint gets in there and causes charging issues. Also, you can do a factory reset to make sure its not a software bug.
If the problem persists and if you still have warranty you should contact Samsung.