Different voltage differences for a UART signal (when tested from one microcontroller, vs when tested between two microcontrollers)
Based on your description, you have connected MSP TX output pin to PIC pin 69 (RP4), and expecting it to be U1RX input pin.
However, based on your code, you have configured PIC pin 69 (RP4) as U1TX output pin, and pin 70 (RP3) as U1RX input pin.
So the two TX outputs are connected together, and the scope measurement proves it.
Two CMOS outputs driving a same node results in intermediate voltages of about half the supply voltage, and the output being exactly half is explained by output drivers not being equal in strength or external series resistances. PIC board output high driver is stronger than MSP board output low driver, so the MSP board output can only pull the voltage down to 1.9V.
Just change the wiring to connect MSP TX output pin to the actual PIC RX input pin, P70 (RP3), and MSP RX input pin to PIC TX output pin P69 (RP4).