I do not know implementation details in Android or K-9 Mail.
But:
Sharing of multiple files was possible until last K-9 update in 2025-05 to K-9 V10.
I guess something changed in K-9 V10 regarding sharing interface.
Sharing of multiple files is possible with most of the apps i use.
I used this functionality (sharing of multiple images to K-9) several years until now.
For understanding my use case i add some details (complex scenario):
Images are usually very large in terms of file size. K-9 has no feature to resize images automatically. So i use an app =Imagepipe // see https://forum.k9mail.app/t/enhancement-request-resize-attached-photos-before-sending/7874.
But beside images i used sharing of multiple files for many years also for p.e. pdf-files to mail with K-9.
I hope that there will be a change in K-9 V10.