Fabric melting spider juice
Some spiders are known to eat their own silk when breaking down a web and reconstructing it (https://en.wikipedia.org/wiki/Spider_web#:~:text=It%20is%20common%20for%20spiders) or even stealing the web of other spiders and eating it (https://frank.itlab.us/photo_essays/papers/Argyrodes_kleptoparasite_spider.pdf). It may very well be that some types are able to digest various silks or other fibres. I'm not aware of any studies that test whether a spider could digest - say - wool or other natural fibres.
If your sheets were silk or other natural fibres, it's feasible that their digestive juices were strong enough to break down your fibres. Who knows, maybe their digestive juices are even strong enough to break down synthetic fibres (unlikely, but I don't know any study that has looked at that).