Yeah it happened to me, that's why I hate assist achievements. I had a situation where I did get an assist icon, but it didn't count towards the "collaborator"

So I got 3 assists which was clearly visible when I opened stats with tab button, but at the end of the battle I didn't get a collaborator reward. 

Also I noticed that if I only damage mods I don't get assist at all so maybe that is your case 

Thanks! The issue might indeed be mods related since now all bots have mods, too ( they didn't earlier at my level). 

The possibility occurred to me but I was hoping it's not the case. It indeed makes earning assists unnecessarily difficult.

"but at the end of the battle I didn't get a collaborator reward."

Unless there's a bug, this could be for a couple of reasons:

1. The assists weren't by a single mech.

2. The assists were in different rounds of a match.

I forgot to mention, I did keep that in mind, wrote down assists I had with one mech in 1 round and didn't shoot in 2nd. (Didn't die in the 1st and hid in the 2nd round so I can test this) because I suspected this had happened before, and I was right. It was definitely a bug

Here is an additional observation about the orginal topic. If an enemy mech has mods, and if I fire at the mech, destroy the mods and then continue firing at and damaging the mech, then if somebody else kills it I don't get the assist. But if I start firing at a mech whose mods are already gone and somebody else kills it, then I get the assist. 

It's not 100% consistent but close. It would be nice to get the assist if we had a hand in (aka "assisted in" :-) ) bringing down the mech, mods or no mods.

It took me four rounds last night (3 5v5 and 1 CPC) to get one assist and I was really trying to get assists. I would take my target down to near zero and let a teammate get the kill, no assist registered. It didn't seem to matter how much damage I did or when. If I had tried that in the past, I probably would have had 15+ assists per round.