I have spent several hours today to get N1MM and WSJT-X set, using an USB cable between PC and IC-7300. The funny thing is tat this works intermittently. I might work for a QSO or so, and then no more PTT, as you describe.
The radio works OK when running WSJT-X standalone.
There might be an error in N1MM settings. I have set in Config>Hardware:
Speed 115200, N,8,1, Always Off, ALways Off, 94, 1, 15, PTT via Radio Command SSB Mode, PTT via Radio Command Digital Mode, None, Normal..
In short periods I had the WSJT-X operating, but then SPLIT appeared when TRX waqs keyed and went off when in receive. That happened never before when running WSJT-X standalone.