I usually roll my eyes when I read about how this particular brand of vehicle requires that particular brand (usually OEM) of part or it just won't work "right".... but your experiment is consistent with my experience with thermostats. I only buy thermostats from the dealer. I bought a "fail safe" tstat from Canadian Tire a few years ago because it was the only one they had in stock. The parts guy said "those don't work very well, you should probably order a different one in". I thought it was silly, surely it would work in my minivan. Well, it overheated within days of installing it, and the temp would nearly redline every time before it opened on the first run of the day. I promptly tossed it put in a Mopar one, and it's worked perfectly for years. For that matter, the mopar one I took out was working fine, but it looked original so decided it should be replaced while doing some other service. Go figure.
2002 Subaru WRX Sedan WRB (314k miles and counting)
2002 GC Overland (90k miles and fingers crossed)
"Bought, Not Built"