I guess the amount of rotation needed between “airtight seal gets broken” and “lid can come off” is fairly short for these thermos. If the difference is, say,

I guess the amount of rotation needed between “airtight seal gets broken” and “lid can come off” is fairly short for these thermos. If the difference is, say, a full 360° turn, pressure will get relieved before the lid can come off. See also https://news.ycombinator.com/item?id=48006887 https://news.ycombinator.com/item?id=48006887. Apparently, many bottles have discontinuities in the threading to allow for that.