Using Tomboy notes on Linux , Windows and OSX. On Android I am using Tombdroid note application.
For synchronization with the Nextcloud I am using owncloud application “Grauphel - Tomboy sync server” ( https://apps.owncloud.com/content/show.php/grauphel+-+Tomboy+sync+server?content=166654 ) .
It is working fine on the Nextcloud 12.