How do I avoid microwave-safe covers melting in the microwave?
I'm no plastics expert but a general rule of thumb is the softer the plastic initially, the more likely it is to melt (I know that sound too obvious, but hey;) Something to do with thermoset vs thermoplastic.
The trouble with plastics being advertised as microwave-safe is it is a limited claim. If you microwave a bowl of water covered by the lid, then you're fine… forever.
However, if you get food, especially fat, on it then that will create a local heat spot far in excess of what the plastic can stand.
I've found that it's not worth spending money on heat-proof covers when in fact far more heat-resistant plastics are readily available - for free.
Your take-away food will come in one - both container &amp; lid are quite likely to survive being nuked with a really greasy curry inside, with little ill effect except perhaps some staining.

Likewise, those supermarket dishes containing ready meals are even more resilient. I have some of those still in use from probably 20 years ago. They can get a bit bashed &amp; the edges will chip after a time until you decide they're no longer worth keeping.

They all go through the dishwasher with no worries too [so long as you make sure they don't flip over, they're very light.]
I have actually been known to buy supermarket ready meal products like this just for the dish ;)