Back to OP question: Our Tecma does the same thing and resolves the same way; usually one reflush, occasionally two. The trouble shooting page in the manual suggests the discharge hose is pulled down below the level of the toilet nozzle and is causing the water to siphon out. Seems to me if that were the cause, it would not be intermittent. Perhaps sometimes the discharge is ‘extra’ thick or heavy that it doesn’t get pumped all the way to the black tank? (And the hose isn’t completely supported end to end and so gets pulled down.)
All I know is the issue is easily resolved with an extra flush or two and only becomes a nuisance when we go to use it and didn’t realize beforehand the bowl was dry.