@D8vid Moved that discussion here
TLP has moved to using Core’s new Thumbnail logic as described above and in that topic… This has great benefits for stability not least because the plugin no longer has to carry this out itself so there is less risk of a breaking change during a core upgrade.
Thumbnails are a Topic only concept. However if you install either the TLP plugin or the TLP Theme Component Sidecar you can manually choose which image from all the posts gets represented so long as the image fits the filter criteria.
Unfortunately that means a thumbnail is not maintained per Post.
This logic is almost completely part of Core now so it’s not something I have direct influence over. I do not intend to expand on the core logic wrt to Posts. It’s beyond the scope of the plugin.