I have a PDF TOC that consists of 3 other TOCs that have been merged into them. I have found the following behaviour (I will use "printed" as "appears on the PDF pages", to distinguish between the TOC in RH versus the TOC as it appears on the PDF pages):
The contents (topics) of the TOCs that are merged are printed in the reverse order of the merged TOCs, meaning the topics are printed in order TOC3 - TOC2 - TOC1. The order inside TOC1, TOC2 and TOC3 stays correct.
The TOC references on the printed TOC page are in the correct order (the same as in RH), TOC1 - TOC2 - TOC3. This results in a very strange page numbering, where the page numbers "restart" after the last page. See screenshot below: page 6 (TOC2 start) comes after page 87 (TOC1 end), and page 4 (TOC3 start) comes after page 14 (TOC2 end). The numbering at the right corresponds correctly with the pages on which the topics are actually printed.
The footer of the last page shows Page 88 of 87. Hurrah, a bonus page 😉
Anyone else have problems with merged TOCS in PDFs? I could of course copy the TOCs instead of merging them, but I'd rather not if it's not necessary.