I use C&C thread , and lots of other brands . Is it an older thread? ... thread does loose its strenght over time.. do a break test against the spool you are not having trouble. Take about 12 inches of each and tug on each .. which breaks with the least stress?
It is always possible you got a bad spool with bad thread or a literally a bad spool , one with a burr on the edge so as it unwraps off its getting frayed as it rubs on the edge of the spool.