Mobs disappear in the Nether Portal.

it's desperate: I empty an entire Nether fortress by transporting all monsters in Minecarts to a Nether portal. They usually go to the portal well. But they never arrive in the overworld. Even if I follow them through the same portal, they're gone.

I turned the portals off and on again, no success.

Strangely enough, some portal pairs work quite normally. So there seem to be parameters that I cannot influence and possibly arise from an error.

Before the Nether update, it also worked normally.