Default Editor Changes Don’t Save

Hi, I installed Swift Performance on 3 of my sites. All three are having issues where any edits made to the title or featured image using the default builder don’t save. They revert back immediately after clicking update. I reached out to my host (Siteground) and they were able to replicate it but unable to figure out why and how to fix it.

I have a few other sites that use the same theme and plugins, but I hadn’t installed Swift Performance on them yet, and they are working fine.

Everything is fully up-to-date on all my sites, and I tried uninstalling Swift Performance but it didn’t solve the issue. I also added the Classic Editor plugin and the issue was resolved, but I don’t want to leave that plugin there indefinitely; I’d rather figure out the specific cause. Thanks for any assistance!

The page I need help with: [log in to see the link]