Using Woot v1.12.2-0.0.5
On Multiplayer, I am using Spongeforge-1.12.2-255-7.1.0-BETA-2831 currently
Note: This works fine in Single Player.

I didn't know if I should report this to you or to SpongeForge, so I'll see what you think first.

So I made a tier 3 altar for enderman, provided it power, and the gui looked like this:
https://gyazo.com/a5a11e28ca3241e9d25e18c285f9aac0

But when I put the Looting I upgrade on the altar, the drops stopped coming and the GUI shows no drops at all. In Single Player it doesn't do this:
https://gyazo.com/7de84fd19fa31fe6583b8f37050f8a30
The setup: https://gyazo.com/6b7e52f1a7b8f3ba2fc5f3ca5a160140

It appears the other upgrades are working though.
I'm going to be updating this mod and spongeforge in the next 24 hours or so. If any other changes happen I'll update this post. Just thought I'd report this to you in hopes for a fix or something.

It is a long standing issue with Woot and Sponge I'm afraid. The below Woot bugs contain more info and links to the bugs raised with Sponge
#288 #139

Aww that sucks. Maybe one day they will finally smooth all the bugs out. HA. Thanks anyways!