Typically we see this repeat failure on tractors that their clutches are abused. not so much excessive slippage, but from what I have been able to gather from the tractor users was that the tractor was placed into a shock loading event. For instance, driving into a pile of gravel at some speed and getting the wheels bouncing/spinning usually in higher ranges. Also, rust can build up between the input shaft and the disc, which will also wear the two parts. Once it wears a little, the disc is now shock loading the shaft every time it's engaged and disengaged. Snowball effect, the end is near. Rust is more common in humid and/or salty areas. The bellhousing is vented to allow hot air out and cooler air back in. As another poster mentioned, really the only time the clutch is used is to stop in an oops situation. Most of the time the shuttle lever is moved back and forth to engage and disengage the hydraulic clutch packs. That is why it was called a "glide shift" transmission...smooth shifting. Really it's a very similar setup as an M series hydraulic shuttle style transmission which has been pretty stout to my surprise. There is no main clutch on those, just a set of hydraulic clutches actuated by the "clutch" pedal.
Your tractor doesn't have a lot of hours, so I'm guessing it may sit quite a bit? Just guessing.
As for the repairs themselves, the warranty statement specifically states that clutches are not warranty items. Page 6, IIRC. The other issue is the cost. The L35 is a royal pain in the backside to split and replace the clutch. The loader has to come off, and it's not a quickly detachable loader. In order for it to come off, the entire rops and canopy has to come off. Then the backhoe. Then the fenders. Lower subframe. From there it isn't too bad but it's just really labor intensive. I've done several and I can usually do them in 4 work days if I work at normal pace being careful as to where I am putting the removed nuts and bolts (hundreds of them), pins, parts, etc. I have hurried and done one in just under 3 full work days but I hate to hurry like that. Sometimes the hurry gets you forgetting a part, a part that usually is in the middle of the tractor requiring some disassembly to get to it...so the time you just saved ends up getting eaten up plus a little.
The disc is softer than the shaft. To replace it, disassembly of the transmission is required. Even more fun (and money) but you already found this out.
Yours was all done recently with the updated parts. The original part numbers are NLA. I do not know what they changed with the new parts, they don't tell me. They might be a different color paint on them, but the originals part number did change. The clutch parts are similar if not identical to an L2900/3300 but the engine has a little more power and the machine is quite a bit heavier.
On the bright side, some of the local grave diggers had bought the newer (at the time) L39's and they all say that the L35 would outdig the 39's. I don't know, I've planted people with L35's and L39's both and I can't tell a big difference, but I don't do it every day. Couple guys keep checking in with me looking for a good used L35, so there must be some demand out there for a low hour L35.