I cant say really what is wrong but Ive used 8-10# braid on a number of spincast/underspin reels for years. zebco33, daiwa goldcast us, omega, pflu 1806 us, a Synergy micro underspin. My 7y/o son yesterday was throwing the Pflu 1806 w/10# braid no issues throwing very nice 40 feet or so only a small bobber. Ive used it for bass on occasion and with just nightcrawler it goes very well.
My only guess would be that by chance could you be getting the subtle braid caught in the threads of the cover as you put it on? This would cause it to get stuck and too might cut the line at the same time. This why even with light power cast the line breaks.
Another tip if you still want to try your braid again get Reel Magic or KVD Line Conditioner take the cover off spray the line put cover on carefully. You can use this stuff for any of your other lines you have to help.