Note that this line does not initialize lastPrint. It would be wise to initialize it to zero, because it will otherwise start with an unpredictable value.
static uint32_t lastPrint;
The ATmega328PB is new and significantly different than the ATmega328P on the Uno, although it is supposed to be backward compatible.
Perhaps Minicore does not fully support the former. Try it with Pololu's libraries instead.
Also, post on the Pololu forum. The engineers there are very responsible and will take notice. But first, fix the initialization.
I suppose it could also be a brown out if the power supply is overloaded or otherwise performing poorly. It seems the Pololu A-Star 328PB (5 volt version) has a threshold of 4.3v. (If you have not replaced the bootloader which may also have changed the fuses). The Uno is more tolerant with a threshold of 2.7v.
Thanks for your support, meanwhile I confirmed the board is resets in a periodically. I use also Pololu AVR 2.1 programmer which has the option to supply the board and this option is used.
Today evening I will try with an independent stronger power supply.