just noticed tonight that both of my lenses are cracked and look kinda melted! I had replaced these new with OEM lenses I bought a few years back, is this common?

Many of the aftermarket tail lights say OEM but that does not mean they are any good. You should see Valeo printed on one of the lenses if they are Volvo.
Dan

Are you sure it's really OEM? There's a lot of crap aftermarket plastic at much lower prices than original Volvo. If you didn't buy at your local dealer, for example got it on eBay, then it's not surprising.

On the other hand, are you by any chance using higher-than-normal wattage taillight bulbs? I've seen 35W and even 50W bulbs on the market as substitutes for normal 21W bulbs -- are you using any of these? The greater heat might account for the melting.

Otherwise, on genuine OEM lamps I've never seen any damage, over many years and on my many cars, that you describe. Cracking and leaking (probably UV damage, etc.), but not melting.