It would have to wipe the memory of everything from that time onward, not just memory of the flash itself. I would also need to be immobilized from before the flash to after the memory alteration otherwise I would almost certainly notice a difference in position, and also tamper with my time sense so that I don’t realize that my memory only covers 5 seconds since entering the room instead of 10 seconds or something. But yeah, someone who can make a basically perfect copy of me can reasonably do all these sorts of things as well.
Rationality (e.g. Bayesian updates etc) as usually defined doesn’t work in the presence of mind alterations (e.g. implanting updates that are not rational), so that makes it rather difficult to develop criteria for what one “should” expect.
If you define it as “winning”, then expecting always to be the original when one has no evidence to the contrary always wins for the original. The scenario only asks about the original, and so this is a winning strategy.
This doesn’t actually hinge on whether they are conscious or not.
Treating them like property when they are not conscious, but are very good at mimicking the behaviour of conscious beings that do not like to be treated like property, could lead to disaster in exactly the same way.