I’m with you, it’s confusing. But I think what it means is this:

The study ran for 90 days. Non-sterol bees had stopped doing bee sex by then. Sterol bees were doin it all the way up to the end of the 90 days - and then the study ended. We can therefore assume they wanted to continue having freaky beedsm sex for even longer.

Some were observed brooding for up to 12 weeks!

one group continued to the end of the study period, the other group had stopped by the same time

or, one group stopped doing a thing, and the other group didn't show signs of stopping

Gotta be AI bullshit. But I'm reading it as, group A never stopped while group B stopped breeding at the end of the period.