It shouldn't matter what method you use to strain the grinds. I used the french press a lot when I was first experimenting with formulas. The batch will have more strength if you brew it for 24 hours. There is never any bitterness in a cold-brew. The strength is also impacted by the ratio of...
I've been cold coffee brewing and selling the brew at festivals for two whole years and never experienced anything like you just described. Is this repeatedly happening? Perhaps it was just some fluke. Don't give up making cold brew. It's so superior for iced beverages. Toddy sells a brewer...