The Rabbanut courts have become experts at reversing important decisions that affect lives.

Various rabbonim and courts have reversed, and attempted to reverse, conversions. The conversion reversals are already old news. Recently the rabbanut courts have been trying to reverse the granting of a divorce in Tzfat - the Supreme Court might not allow them to for procedural reasons, but the Rabbanut court has already ordered it reversed.

And now, a rav is saying that immersions in the mikva by women in his city might have to be reversed and the women would need to re-toivel.

Chief Rabbi of Kiryat Motzkin, Rav Dovid Meir Druckman, has decided to fire an Ethiopian woman who was recently working for the city's religious council in the position of a mikva attendant. It seems Rav Druckman raised doubts as to this woman's Jewishness and if she went through a proper conversion (as Ethiopians from Ethiopia were required to undergo conversion lchumra). Because her Jewishness is in doubt, it renders all immersions performed under her watchful eye as invalid - or at least in question..

The Rabbanut requires women to toivel in the presence of a balanit -a mikva attendant.

Rav Druckman denies that his relieving her of her duties had anything to do with her being Ethiopian, but says she had been hired without first getting his approval while many other women have applied for these jobs and had been tested by him, yet she "jumped the line". He also denies having made an issue of her Jewishness, but explains that because he does not know her that was one of the questions he asked.
source: Walla

Whatever his problem with the balanit, reversing tevila already performed seems crazy and against halacha. Either the woman toiveled or she did not toivel. The need for a balanit is important, but if a woman toiveled without one she is considered immersed and tehora, unless there was a known problem (such as hair not going under the water, among other possibilities). The Rabbanut stance is that a balanit is required, but the city councils have decided to not require it if a woman doesn't want one present.. which means whatever the Rabbanut stance, it is clear that it is not an absolute requirement for the kashrus of the tevila.

So even if this woman was not a Jew, which is a ridiculous assumption with no basis, the immersions would all have been kosher. I do not know -can a non-Jew be an attendant at an immersion? The job is completely functionary, and not an issue of testimony, so I don't see why not, but I don't know if it is ok or not. If yes, then what is the issue here, even if she isn't Jewish (again, a ridiculous assumption to make)?

And if the issue was not her Jewishness but just that she was hired through improper channels, why would that render all the immersions invalid retroactively (even if that were possible)? Because she had employment issues her supervision of immersions did not count?