I've definitely seen this as well. I've noticed that if you resave the CSS file, it'll refresh the design view and reapply, though after some time the same issue occurs and you have to repeat.
I always seem to have this when I add new styles, and it seems to go away after a while on older ones, so I wonder if it's some kind of caching issue.
I wonder if you could have a flag to run the design preview in Incognito mode within Wappler which should avoid caching issues? I know you can do this in Electron by using a null partition for the browser window (as Wappler is built upon Electron shouldn't be too difficult?). Just a brain fart.
Made a new one on v7.2.1, you can match the exact hour and minute with the log.
Created a project, and 1 minute later, style doesn't apply, although on the editor is there:
I was able to reproduce it and found that the design view is caching too aggressive. When making the change the design view is live updated but when you add a new component in the root of the page it reloads the page getting the cached stylesheet. Will work on a fix.