Currently you have to hold SHIFT+RUN to (force) start the 2400 when it is synced to MIDI Clock.
To start the machine in recording or play mode should always be just RUN or RUN+REC regardless if it is running free or synced to MIDI Clock.
That is what people would expect without looking in the manual.
From the manual:
When clocked externally, the Run/Stop key is disabled, but Shift+Run/Stop will force the S2400 to run. Once a MIDI start message is received the external clock takes over. Pressing Run/Stop+Rec/Edit (which normally starts recording) will arm the record function, so that when a start message is received, recording will start with it. Pressing Shift+Run/Stop+Rec/Edit will force the S2400 into record mode.
I guess arming the record is more a special function and should have a lower UI priority than forced record.
For that reason forced record and arming the record function should be reversed, e.g. press Shift+Run/Stop+Rec/Edit arms the record while Run/Stop+Rec/Edit forces the record function.