This is indeed a limitation in the FT8 (or FT4 or JT65 or JT9 or ...) protocol, and not in N1MM+, but it is not as simple as a character count.
FT8 messages are not encoded in ASCII. The data payload in an FT8 message is only 77 bits, and yet it can accommodate a message like KA1ABC DL1ABC JN68, although that message looks to us like 18 characters. In fact, that same message can also be sent in JT65, which has only a 72 bit payload.
The bottom line is that only standard call signs can be sent in the standard message formats. Compound call signs fit into a much more restricted set of message formats, and those don't include grid squares. Consult the WSJT-X documentation (chapters 7 and 17) for more information.