Separate membership from the visible wrapper
A related collection can currently contain only one available movie. Kodi has an Include sets containing a single movie setting, so seeing the movie directly in a list does not by itself prove that its set relationship disappeared.
Review the specific library item
Inspect its set membership and then the relevant movie-set display setting in the installed version. Record the current view and skin. Do not rename the set or rescan the whole source merely to create a more conspicuous group wrapper.
Consider the intended outcome. A household may prefer direct access to lone items; an administrator maintaining a collection catalogue may want the group visible even before more members are added. Either decision should be explicit rather than inferred from an unfamiliar interface.
Keep a small before-and-after note
After the setting change, review that one set and an ordinary multi-member set. Confirm that navigation still fits the library's purpose. Keep the previous value so the presentation preference can be reversed without changing the stored membership or source files.
Questions answered
Does a missing set wrapper prove the movie lost its membership?
No. Review membership and the single-item display policy separately.
Should I edit media filenames to make the group visible?
First inspect the library relationship and display setting; filenames are a different concern.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.