[Quoth]

Though in theory a non-linear TOC is possible, it's a stupid decision by the formatter/designer.
Compare a paper book. The Contents page at the start is always linear. If you want the content grouped in some non-linear way you put specialized indexes.

So the system TOC should really be strictly linear, even if non-linear works on something. The "grouped by" "contents" is a special index and thus should be inline pages at the start or end of the book and get an entry in the system TOC.

I think many physical eink only support linear, though a Kindle isn't epub, you usually upload epub file to KDP.

All kinds of mad things work in the Calibre viewer that shouldn't be in widely compatible ebooks, because it's a custom use of a web browser. I only use it to check links and that the (system) TOC is matching the original Wordprocessor inline automatically built contents page, which unsurprisingly is always linear!