Thank you, I looked into it. Here is a reply I made in another thread:
By design, Greasemonkey scripts necessarily trigger on page load. However, KES initializes its own mutation observer and watches for events that change the DOM, but does so in a granular fashion so that only those of relevance (changes to the thread index and comments) are propagated. It looks like when turbo mode is enabled, the entire DOM up to the HTML tag is replaced at once, so our more granular approach is missing this change. We need to watch for basically all mutations now and filter them accordingly.
After making the necessary changes, it seems to be working again as intended, but some additional refactoring is needed, and I haven't checked the implication of this on specific scripts/add-ons yet. It looks like it should be easy enough, since the majority of functionality I looked at was working again when using a more permissive observer.
I haven't added these changes into the release version of KES, and given that turbo mode is listed as experimental, I don't think we can support it immediately over a more traditional page load structure, but at least I am aware of the issue now and can start refactoring it. Hopefully it is not problematic.