As said I can reproduce this in latest b3 for RAW - but now tested Enterprise and that is not affected, seems to be RAW issue only.
Sorry for this situation - as written I’ll open an internal ticket for this!
BTW: it seems to be a user (object contact) issue in RAW - if you have an admin user who’s just contact for some hosts and you change the “visibility” setting in this admin user’s profile, no hosts get shown anymore. Truly, all hosts are still available in setup module because of the admin nature, but not in monitoring.