At the end of Ant-Man and the Wasp, before they’re dusted, Scott goes into the Quantum Realm to get materials so that Ghost can survive. Hank, Janet, and Hope are observing. Janet even gives him some tips! Everything seems fine until, you know, Thanos. But if Janet was so scared of the Quantum Realm and Kang, why let Scott go down there at all?
The answer here depends on how literal you want to get. The real answer, most certainly, is that the Marvel Studios team hadn’t conceived Quantumania yet so there was no reason for it to be an issue. Taking that reality off the table, maybe Scott wasn’t going down deep enough to alert Kang, or Janet felt it was a short enough trip that no one would notice. There’s no way to say for sure, but that’s our idea.