I have had a FM-9 in the box for longer than I should admit and finally got it out.
Initially ran 10 round belts through it to ensure function but once I hooked up a larger belt or tried to run a 150 round bag the belt would pull apart.
Specifically, the belt inside the feed chute would pull apart by stretching the single arm of a link out, causing the belt to separate.  This was consistently at the point between 3 rounds or so and the rest of the belt.
The links and upper were purchased in 2019.  Ammo was S&B 115gr.  Semiauto, standard strength buffer spring and buffer.
I've emailed Freedom Ordnance but thought I check here first.
Any help would be appreciated.

So even laying the belt on a table or something it comes apart?
I haven't had that issue with mine.  I also don't really care for the box.  I've shoved some into a 556 nutsack instead and it worked fine.

No, the belts are fine until I start firing the gun.  
It only happens when firing a longer belt or pulling it from a pouch.
My guess is that it is a violent cycle advancing the next round against the weight of the belt/pull from the pouch.
I've put in a red springco buffer spring and a heavy buffer weight hoping this will slow down the action but won't get to test it out until later this week.
If it's not that then I may have a bad batch of links that are stretching out with the force of advancing the belt.

Below is a link to a pic of a bunch of the deformed links and a single link that is normal.

https://www.flickr.com/photos/198585921@N08/52966841432/in/dateposted-public/

I like slow and smooth.
I know it is straight blowback but this is the best I could do:
I think some of the energy it takes to pull the belt does make it a little smoother than typical Colt 9mm straight blowback.

That's what I'm going for.
I've got an M16 RR inbound sometime this year and will be picking up an MCR as well.

May be that something is binding with the heavier belt/nutsack and causing the link to give way

Did you get this resolved?

If you are rolling your belt in the box or pouch try folding it in instead,it will feed easier with out having to unroll the heavier mass of the whole bel.

Update:  I got in touch with Josh and he is going to help figure out what the issue is.
Glad that the company is continuing to be available and support the product, even though I have had it sitting a closet for literally 3 years!

Thanks Josh