Spawning can only happen out of line of sight.

Monsters already on the board block line of sight.

Is it possible to spawn behind a current monster instead of always having to do it "around a corner"?

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For
purposes of spawning new monsters, other monster figures do not block a hero’s line of sight .

So no, you can't.

And to get the complete picture: Figures never block LoS for spawning purposes (not just monsters).

Well that answers that. Thanks.