Charges fine with new semi-charged battery, but doesnt when depleted

Symptoms include:

1. a brand new battery discharging like crazy. 50%>0% in like 2 hours while idling at the lock screen after boot, with all connectivity turned off except mobile data. while the screen is asleep.

2. unusual and somewhat alarming heat build up somewhere under the shield closest to the display connectors. top or bottom side, i am unsure. but not hot enough to trigger the temperature warning.

the phone is still capable of charging a battery when a semi-charged battery is installed. and it does so, seemingly without issue. however, when you let it die, it DIES. it displayed the dead battery symbol for maybe 30 seconds and then it never came back on its own. connecting it to a charger does not bring it back to life at that point. the phone wont detect the cable. its almost like theres not even a battery in it.

i have tried:

1. new batteries

2. new charge ports

3. attempted to get a power reading with this little charge port tester thing that i have. it shows 5.13V and 0.08A, but i got the same reading on another iphone i know works properly. not sure if my tester is just a piece of garbage or what.

any idea what i might be dealing with? it just seems like theres something bad on the board causing the battery to drain like crazy. im not really all that knowledgeable about board level repairs.