@slaff , thank you for checking the issue and the explanations, I appreciate it a lot. I did check all the header files for information about the code's functionality. In fact, one of the first things I always do is to generate docs via doxygen.
I've delved deeper into this, and it turned out that the issue was in the way I parsed the received data. I'm using the delegate-callback method, with my own SerialDelegate class parsing the data in an onData callback. Before the HardwareSerial refactor I received a callback for each new received character, also when strings were sent. Now, after the refactor, I receive only a callback for the last received character, seemingly missing a lot of data. So now, upon receiving a character, I read the serial stream and check for the message that I expect to receive. That's now working perfectly, also when swapping the serial over to GPIO13 and GPIO15. Thanks