Do you have a source that explains this? My reading of the plain English rules suggests that if you withdraw (to pay a fee), you can’t put that money back, unless it’s a flexible ISA. (I read that Freetrade’s initial offering will not be flexible).
Even if it was flexible I’m still not entirely sure how fees are regarded. I believe that companies that have flexible ISAs recommend having the separate account for fees, so I assume there’s still some kind of issue.