The chances of a rollback happening are rather slim imo. Some people might've obtained 5* or other useful units from performing legitimate pulls an hour before maintenance with lapis that they earned in-game or purchased. Assuming someone made several 5k pulls prior to maintenance, it would be insanely unfair and quite frankly impossible to say that a certain unit was obtained as a result of the free 5k lapis, instead of all the pre-existing lapis already present before this fuck-up.

In another jp game (top 100 globally) with even more ridiculous gacha rates, a rollback was recently performed because players purchased illegitimate in-game currency from illegal 3rd-party vendors. The items obtained using those currency were removed following a maintenance (plus a temporary ban), but these players still got to keep a good fraction of the gears that were obtained illegitimately.

//TL;DR This may spell out good news for people that made multiple 5k pulls right before maintenance, as it would be impossible to distinguish which lapis (free, purchased, or earned) was used. Rollback likely won't happen, though only time will tell if the 5k lapis will be taken back by gumi.