@Manu-el ,
Sorry, I misunderstood your question originally (because I didn’t remember that the sync functions had buttons on the toolbar, so my mind immediately went to the CustomizeToolbar plugin, instead of thinking of the built-in toolbarIcons feature).
In v8.4.2-v8.4.9, Notepad++ was not correctly tracking whether horizontal/vertical synchronization was possible, so it wasn’t changing those icons, and thus, it always used your custom sync-vertical.ico and sync-horizontal.ico
However, in v8.5, Notepad++ fixed that tracking and started using an enabled and disabled version of the builtin icon, but the developer apparently didn’t think about adding the ability to have sync-vertical_disabled.ico and sync-horizontal_disabled.ico – so when the sync are enabled, they use your custom icon, but when sync is disabled, it defaults to using the builtin icon.
In order for it to be implemented to look for sync-vertical_disabled.ico and sync-horizontal_disabled.ico when the sync is disabled, someone would have to create an official feature request (as described in our FAQ), and then the developer or one of the other volunteers would have to implement that feature. If you do submit such a request, please paste a link to the request in a reply here, so that other interested people can follow the status of the request.