Very strange issue here: NextCloud 18.0.1 and moste recent Mac desktop clients. All clients do sync correctly except one. When we duplicate a folder there (no matter what position), only the top level of that new folder gets sync’d. Its deeper levels don’t until we create an empty dummy folder in each level. Could it be related to APFS handling of duplicates?
Why isn’t it on the other clients? Did you check the logs for some information? Same version of the client? Please use a current version of NC 18 as well, some bugs could be fixed already: Nextcloud server changelog