Hello,
When using a prop manipulator to detect a prop being damaged… any weapon with ammo type “bow” does not register the damage when using “On Damaged Transmit On”
Here is a video demonstration (I tried with various other bow types also)
And here is a image showing the simple setup here
Thank you 
So there might be a bug as you said, but maybe as a workaround you can try replacing the prop_manipulators with giant triggers set on transmit on damage, I never use prop_manipulators since they were really buggy for me.
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