Hello, thank you for your reply! Yes, I am using Nitropack for optimization. However, I need to clear the cache many times to fix this issue. I just received the reply from their support:
As there are a lot of changes being made on the site, the cache is being purged regularly which is why the issue appears for a few seconds to a few minutes, while a non-optimized version of the pages is being served.
In the way, the problem is on the unoptimized version only. It’s less likely to be related to NitroPack, so I’d recommend you to check with your hosting provider or a developer who’d be able to help you fix the unoptimized version, so you’ll be sure that the site will be looking fine even when the cache is being purged.