Electronic – Printing variable characters to UART does not work, constants work fine

I have a rather strange issue with XC8 on a PIC18F27K40 microcontroller. On a PIC16F1778 it works. I have defined:

void uart_putch(unsigned char byte) {
    while (!PIR3bits.TX1IF);
    TX1REG = byte;
}

When, in my main loop, I call uart_putch('a');, this works fine. However, when I define const char c = 'a'; and call uart_putch(c);, it does not work. It prints something, though not an a – I think they are 0x00 characters, which I get from hexdump -x /dev/ttyUSB0. This is not an issue with the serial port on my computer; I looked with a scope and the signal is different (left works, right does not):

The code is simple:

void main(void) {
    init(); // Sets up ports and UART control registers
    while (1) {
        uart_putch('a'); // or c
    }
}

What does not work either is using any of the string functions (puts, printf, etc.), which I think is related – so in this question I made a minimal working example with characters.

The generated assembly when I use a variable c has:

_c:
    db  low(061h)
    global __end_of_c

_main:
    ; ...
    movlw   low((_c))
    movwf   tblptrl
    if  1   ;There is more than 1 active tblptr byte
    movlw   high((_c))
    movwf   tblptrh
    endif
    if  1   ;There are 3 active tblptr bytes
    movlw   low highword((_c))
    movwf   tblptru
    endif
    tblrd   *
    movf    tablat,w
    call    _putch

And with a constant it has in the _main block:

    movlw   (061h)&0ffh 
    call    _putch

I'm using MPLAB XC8 C Compiler V1.41 (Jan 24 2017), with part support version 1.41.

The relevant parts of my Makefile:

CC:=xc8
CFLAGS:=-I. --chip=18F27K40 -Q -Wall

SRC:=main.c uart.c
DEP:=uart.h
PRS:=$(subst .c,.p1,$(SRC))
OBJ:=main.hex

all: $(OBJ)

$(OBJ): $(PRS)
    $(CC) $(CFLAGS) $^

$(PRS): %.p1: %.c $(DEP)
    $(CC) $(CFLAGS) -o$@ --pass1 $<

Any help to get this working would be very much appreciated.