The users have InDesign V16.1 installed on macOS Big Sur (11.4).
They use custom software keyboards (created with Ukelele) to type Native American language characters (Salish via IPA). They use custom fonts that have the required glyphs for the language. OpenType marks are used for composing characters. The fonts are both Unicode and OpenType compliant. Frequently a properly composed character will consist of a primary glyph (such as "K") followed by a combining glottal and a wsuperior. If each key of the custom software keyboard has only a single unicode code point assigned, everything works fine. However, if the custom software keyboard has more than one unicode code point assigned, then only the first unicode code point glyph is accepted and displayed by InDesign. This is consistent and repeatable in InDesign. With all other tested apps (Word, Pages, Mellel, OS Finder, etc) both the custom fonts and custom keyboards work fine (a single keypress, with multiple unicode code points assigned) displays all of the glyphs assigned to the keystroke.