So after like 120 casts/screenshots I got it, not sure if this is enough evidence for anything but i tried.
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You can see mobs are kind of behind that hill, but still cast.
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here we see the mobs have come over the hill in full view, my castbar is gone so im finished casting and no interrupt/line of sight message in my chatbox where it should be.
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This is what a line of sightmessage normally looks like for me.