It’s definitely cache, if the back-end is changed.
Make sure to clear your CDN cache as well, they tend to cache CSS assets for at least 12 hours.
I highly recommend turning off every type of caching until making changes. Some cache solutions tend to remove query strings from assets as well, which makes it extremely hard to clear on every level. (browser, page, minify, CDN)
Best,Ernest Marcinko
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