Unreliabilities with title_sort in AZW3?

Hello, this is my first post, so let me start thanking everyone involved with Calibre development for the amazing, titanic job!

I am trying to set the title_sort metadata parameter for AZW3 ebooks (starting from EPUB), but the results have proven extremely weird. (Other fields and EPUB work as expected.)

With Python API (calibre.ebooks.metadata.meta set_metadata()):
This is ignored. (Works in EPUB. Works for other fields in AZW3.)

With ebook-meta --title-sort:
This is ignored. (Works in EPUB. Works for other fields in AZW3.)

With <dc:title opf:file-as="???">:
This is recognized in EPUB, but is not transferred to AZW3 (using ebook-convert).

With <meta name="calibre:title_sort">:
This works SOMETIMES using ebook-convert.
This never works using ebook-convert --read-metadata-from-opf.

This whole thing is so weird that I really don’t know how to approach it.
I will gladly file a bug report if you think it qualifies as a bug.
I can also take my thread somewhere else, if necessary. (?)

Do you have any ideas?
Thanks in advance!