Several year ago someone offered me an entire bolt of high quality unbleached muslin for a really good price. I have used it a lot. In my experience since I am not a prewasher it behaves very nicely. I use it in backs, and sometimes even in patchwork. When this bolt runs out I will attempt to get the same kind again.

what is the brand of muslin you use

It is Premium Quality Muslin unbleached (I thought it was bleached) by Roc-Lon #405 38" wide

I got it way before the pandemic.

I would test it out by cutting a 12" square and pouring hot soapy water on it in a plastic bowl with a lid. Shake like the dickens and then pour off the water and rinse the same way. Check the size of the block after it is out of the dryer and see if it shrank. Better to know for sure, right?