I would say that it is a question as to timing. If all the debuffs are applied at the same time, there would be one check to see if the are cleansed (not resisted). If they follow a progression of application (one after the other) it should check them each individually which would allow for more opportunities to cleanse pre-existing debuffs as well as those actively being applied.
In addition, note that this is all happening AFTER potency/tenacity check are done, so in the test cases indicated above, it is not specifically the zeta cleanse that you are seeing there. It could be a straight tenacity check succeeding. What might be a better experiment is to attempt to debuff Rey when she has an existing debuff already on her and see whether THAT one gets removed.
