Many of us keep Magnifica for a long time.
OverFLOW are for water flow. Having a huge animal on it will obstruct it to certain degree. Large enough anemone and it will obstruct it completely. Partial obstruction will change water level in tank and in sump. Depends on how you have top off, and how our skimmer level setting in relation to the sump level, this may or may not cause problem. Complete obstruction will cause problem.
Does not matter how you screen your over flow, if there is enough obstruction, the anemone will tear.
It is possible that you don't see what we point out. If this is the case, it is too bad. As human, we don't always have to learn first hand. You may have to learn first hand and that will cost you at least this anemone, possible your whole tank.