I am using Magento 2.3.5-p1, and up until now everything was working fine. However, its been 2-3 days that it stopped using cached files even though I can see in redis monitor that cache is working, also in Page view source, I see all of individual CSS files, previously I had only 3 CSS files, and CSS merging was working fine.
My CSS merge & minify is enabled in admin backend. I am not sure how else I can trouble shoot the problem.
Re: Magento 2.3.5 Stopped using Cached and merged CSS
Standard systems use to name minified css and js as hash of the content.. as described above.. If content changes, it is bug of css minifier (i dont know if in magento) or supply of css to that minifier (perhaps cached css before minifying)
The name of css or js file names are usually derived as hash of the content because the browsers can cache it properly and change it when developer makes a change.
If magento does not change minified name when it changes content it is clearly magento bug. Note that i have not investigated if I can replicate the issue.. i am just commenting on your statement.. i dont believe that this issue needs more update. It needs to be checked by magento developers.