Issues like this with softer plastics aren't uncommon. They're not universal, just not uncommon.
The primary contributor to this are the additives mixed, or compounded, with the base polymer and then moulded into the parts of the kit.
For instance, raw, pure polystyrene is the brittle, clear plastic used for clear parts and canopies. All the colours in kits are additives. The varying hardness we all know exists between different manufacturers and kits are also a function of different additives compounded with the base polymer. Others are added to increase mouldability, decrease shrinkage, etc. etc.
When it comes to the flexible plastics like vinyl or DS this is achieved through additives called plasticizers, adding plasticity (flexibility) to the base polymer. These, like all of these additives, come in a bewildering and ever-changing number of types and effects.
Now this would all be much more manageable if this sort of thing was strictly controlled. But, outside stuff bought to aerospace/defense/nuclear specifications (and paid for on that basis) these things are not terribly well controlled at all. Lots of times the compounder changes the additives to suit themselves (it's a touch cheaper or allows other additives to be inserted, etc.) and other times the moulder wants something a bit cheaper or easier (=cheaper) and they both simply make the changes they want and as long as there isn't a gross effect on the product, no-one's the wiser.
But sometimes there is an effect and it takes time to show up. Plasticizers, especially, have a tendency to leach out of the moulded product over time sometimes on their own, and sometimes in response to environmental conditions like heat, light or the presence of other chemicals. Plasticizer leaching out can leave a film on the object or on objects it touches. It can leave a product stiff, brittle and prone to cracking/splitting. It can also leach out of one product and into another making it look like the other product is "melting" if it is susceptible to that plasticizer.
The softer a part is, the more plasticizer it generally contains. The more, or more aggressive, at any rate. The more plasticizer a part has, the more likely it may be unstable over time.
Commercial compounders don't really check for this sort of thing and make no guarantees. If it happens, it happens and you're stuck. They also don't generally guarantee that their particular compound recipe won't change over time so a part that was good, may be not good for a while and then may go back to being good again. And it's entirely possible that no-one in the chain knows or cares why. It's a good bet that the compound recipe will change on a reissued kit, especially if several years have passed.
All of this is to provide some perspective on why this sort of thing happens and yet doesn't happen to everyone or every time.
Personally, I tend to avoid the flexible products on my models. I've had good luck with old school vinyl track, but avoid the flexible mantlet covers or DS track or soft truck tires simply because I feel the risk of leaching plasticizer is too high and I don't want to take a chance. If a new kit comes with flexible parts I now bag them separately to prevent contact and frequently don't use them in the end.
Here endeth the epistle...