Sorry if this has already been mentioned. i did look before posting.

ISSUE: Vanished players pick up fired arrows. These arrows do not appear in the inventory of the vanished player, but are removed from the environment

How to Recreate:
Fire several arrows so they land on the floor. Walk over them while vanished. Arrows will be picked up, but not appear in the players inventory.
NOTE this only works with the fired arrow entity, and not a normal dropped arrow item.

Players now use this to detect if an admin is nearby, by lacing tight entrances with arrows so you have no chance of avoiding them.

End of Report.

Extra Data:
Also, is there a way to enable some kind of "no-clip" effect when vanished?

I can confirm we've seen this as well.

May i note that the arrow entity we pick up while vanished is the arrow fired by the infinity bow.

Ordinary fired arrows are immune.

Only the arrow normal players cannot pick up is picked up by the vanished player.

I'm not getting the same results, could it be that you're in gamemode 1 while picking up the arrow?

yes, while admining we are in gamemode 1. This is not really something that is practical to change.

The issue is we are being detected, while vanished, by picking up the fired infinity arrow. All other fired arrows from normal bows do not get picked up, nor do arrows lying on the ground.