How to find .emlx and .eml files with Spotlight on macOS Monterey?

I have noticed that our app on macOS Monterey cannot find email files with .emlx or .eml extension stored in directories shared with the app.

I am aware that email indexing on macOS Big Sur has migrated to Core Spotlight and thus can no longer be found via Spotlights. However, a metadata importer for the file formats is available under macOS Big Sur. So if files are in either file format and have been placed in a shared directory, they can be found via Spotlight.

Under macOS Monterey I find that the full text of such files is not indexed. You can reproduce this behaviour with the following examples. Precondition: In one of the accessible directories, for example for documents, there is at least one file with the extension .emlx or .eml.

The following query shows a list of emails in the user directory:

mdfind -onlyin ~ "kMDItemContentTypeTree == 'public.message'"

The following query returns the corresponding email with the filename 20.emlx:

mdfind -onlyin ~ "* == '20.emlx' && kMDItemContentTypeTree == 'public.message'"

The following query already returns no results. A tokenizing of the titles does not take place:

mdfind -onlyin ~ "* == '20' && kMDItemContentTypeTree == 'public.message'"

The following question does not return any results, although the word is contained in the text of an e-mail:

mdfind -onlyin ~ "kMDItemTextContent == 'apple' && kMDItemContentTypeTree == 'public.message'"

Does anyone know if the reason is that this is an early beta version or do we need to prepare for a helper solution, for example with SKSearch Kit?