That tab you are hovering is “special” (because it has a protected domain addons.mozilla.org, there are a couple of them ) addons can not inject content so i assume generating a screenshot also wont work here.

Also the addon wont be able to generate previews if the tabs are discarded, so the tabs need to have been active at least once in the current sessions, otherwise there also wont be a preview available.

Thats what i gather from my own knowlege and from this issue https://github.com/easonwong-de/Tab-Preview-On-Hover/issues/1

tldr;

1. Some protected domains wont have previews (addons.mozilla.org)
2. unloaded aka. discarded tabs which havent at least once been active in the current browser session (restored as unloaded) wont have a preview either.

Hope this info helps.