Very like the dodgy aftermarket VW header tank caps, there are also dodgy Subaru aftermarket thermostats. These can also be sold in the packaging of usually reputable brands, but not perform anything like the correct OEM part. The good Subaru thermostats are not expensive, and the dodgy aftermarket ones sometimes sell for as much, so using them is a false economy. Just use a genuine Subaru part. Subaru thermostats very rarely fail in our experience, and we do not recommend ever replacing them as preventative maintenance, unless you have good reason to, such as you have tested one in boiling water and it either didn’t open or close properly.