It seems like UART Rx is receiving some noise from somewhere...
Please try this: take a magnifying glass and inspect carefully the original UART connector you seem to break, check if the teared trace is not touching anything etc. Just in case take a really sharp scalpel and try to clean around the original UART connectors.
If that does not help you can try to disconnect the mainboard from all peripherals, remove it completely from the case (remove the sim card and anything you can disconnect) and submerge it in isopropyl alcohol for 5-10 minutes -> buy* 0.5L of IPA, pour it into a ceramic plate and put MB there.
This should help remove all flux residues without damaging anything further.
*as it's now used for disinfection IPA prices skyrocketed recently, but it still should be affordable
