TBH, that seems more like one of the samples got contaminated with soap or something. The difference shouldn't take much more time to manifest than the time to discharge, which would at most be in the order of seconds. Also, a tiny sample on a plate would have such low capacitance to ground that just sliding the plate in different ways when putting it under the microscope could cause differences in charge.

The only possibility I can think of would be if the effect wasn't electrostatic in itself, but something the cells actively do - which I have never heard of, but I don't know enough about biology to confirm or deny it - and the field slowly causes changes in them which prevent them from doing it, and they slowly heal from that in the absence of a field. Seems far-fetched, but I'd need an opinion from someone who knows this part of biology to know.

(As I have written before, I don't like the appeal-to-nature-and-the-past "explanations" i.e. "we've always been grounded, so that's what we should do". I want explanations of the mechanisms involved.)