We are using Joomla 4.2.8 together with T4 BS5 Blank template 1.1.7 and T4 plugin 2.2.6.
Today we noticed that filename of CSS Optimization by T4 is always the same e.g. when we update custom.css and new CSS optimized file is generated the filename doesn't change. This means if user's browser already cached it, they won't get the updated file until it expires or they hard refresh their page (not likely).
In our case the filename in '/media/t4/optimize/css' folder is 'd53bc4db7b1dd47f6bd29413e87ef314.css'.
That string should be changed / randomized every time a new file is generated.