G-SAYE is reported as WFU at
Aurigny Air Services Fleet List. It was also reported in the Guernsey Press. Bearing in mind that G-SAYE is a Do228-200 rather than a higher spec a/c and consequently has a lower MTOW (by 500kg compared with G-LGIS and 700kg compared with G-AOUR), this is not perhaps surprising. The interesting question is did it have a higher allowed MTOW when on the German register as D-IFLM and operating to the Isle of Man? There are -201 and -202 versions that have a higher MTOW, and a -200 can be converted to one of those. Maybe (and I'm guessing) a conversion was accepted by the German authorities but not the UK ones.
As to G-OAUR - maybe some of the crews operate both types of a/c and the rostering simply means that the Trislanders were all operating last week. This isn't necessarily implying a fault with the a/c.