I am assuming the the company count it as a continuous duty otherwise you would require the 12 hour minimum rest period. We sometimes have to do airport standbys if for some reason our rostered duty is cut short (most likely unable to do final 2 sectors of a 4 sector day due to delay running past max allowable fdp). I however have never heard of an (in advance) rostered duty as you have mentioned. Sounds like a nuisance but probably quite legal.