Last night a fellow ham and I decided to use FST4W 1800 to see how it works.  It began transmitting and at 180 seconds into the transmission, it stopped transmitting, my IC-7300 transceiver went into receive but WSJT-X software still showed it was in the transmit mode until the sequence completed at 1800 s, then went into Receive mode.  I did this several times always with the same result.  The fellow ham on the other end also had the same trouble and it appears there is a bug in the software.  In fact for every mode that was above the 180 s. (300, 900, 1800) this behavior happens.  120 s doesn't drop out like this because it is below the 180 s. fault.