Hi,
That indicates a missing stylesheet, the styling is not included in the page. If it works for a few seconds, then it’s definitely an obsolete cache issue, the old cache is served where the plugin is not yet active and therefore styles are missing.
Maybe the cache doesn’t notice the changes, try changing this option: https://i.imgur.com/j0xKWh6.png
This may trigger the caches to update as the stylesheet inclusion is different.
Try both file and inline variations and make sure to clear all layers of cache – inlcuding file, asset, browser, CDN, cloudflare etc.. Even if one of them is triggering it will serve old cache content.