I have an old iPad running iOS 9.3.5 (the latest version that the hardware will support). I also use an iPhone (iOS 12.2) an iMac (High Sierra) and a MacBook Pro (also High Sierra).
Notes sync perfectly through iCloud between the Macs and the iPhone. However, the sync seems one-directional when it comes to the iPad. Text changes (including note creation/deletion) that I make on one of the other devices are sent to the iPad, but changes made on the iPad are not sent back to the other devices. This can lead to some very weird text on the iPad, as if parts of the text (rather than a complete note) are being stored locally, and combined with the text that is synced via iCloud. The result, on the iPad, is a jumbled mess.
I am presuming that this is because of the changes made and extra features added to Notes app in more recent versions (in both iOS and Mac OS) that prevents syncing from taking place properly, and that there's nothing I can do about it, but can somebody confirm this? All the results I found from googling relate to the opposite situation: a newer version of iOS failing to sync with an old version of Mac OS, which is not precisely the scenario that I have here.