I just had the exact same issue the last couple of weeks.

Last summer I bought an extra pack of string and left it in my garage with the trimmer. I started using it about a month ago and found all 3 spools were like this. The string itself seemed more brittle than it should be, and some of the wraps managed to squeeze down under earlier wraps so it eor on getting jammed.

So I suspect it was either just a faulty batch from the factory, or perhaps leaving it in my non-climate-controlles garage for almost a year may have damaged it.

I suspect the solution is going to be buying different brands of string to find one that works. Some trimmers have options to replace the head for different styles as well, such as replacing an automatic feed system with a bump head, or heads with metal blades for heavy brush. Something like that may be an option too.