Hmm... A 3chee to my touchee...
The terms and conditions of Economy Singles as shown in 'the manual', which ATOC advertise as now being the only source for such information, and from where info in the Avantix is taken, state that break of journey is permitted on the rail portion.
I'd hazard a guess that the info in the Avantix is wrong, particularly as you could never actually issue one of these through a mobile TIS anyway, due to the requirement to make reservations and produce a sea control ticket, neither of which are possible on them.
Also, in cases like this where there is a degree of ambiguity, with conflicting instructions to staff, staff are urged towards leniency in the passenger's favour.
Should the passenger nevertheless face an obstinate guard who makes them pay more or pay again, TOC customer relations offices will always have to back down and refund it as long as there is still published information which states that the ticket is valid.