RM is based on a relational database called SQLite. Relational databases have a feature called collating sequences that are either built-in to the database or which can be added on to a database. RM uses a special collating sequence called RMNOCASE which it adds on to each RM database (my terminology of “adds on” may not be 100% technically accurate, but essentially that’s what happens).
There is some sort of difference in behavior between RMNOCASE on Windows and on a Mac. This issue has been discussed previously at RMNOCASE differences between Mac and Window If Reindexing is the permanent solution to this difference, then the need to Reindex when moving RM databases between Windows and Mac should be well documented. It may be documented somewhere, but I don’t remember seeing it.