If it is local only and the app you like does not connect to the internet, it is just like any other text edit app on your OS that doesn’t connect to the internet. I see no real problem from an online security POV and unless the encryption it uses has been weakened by passage of time and lack of updates for itself and the OS you are running it on, it should be okay.
But as a rule of OPSEC as I understand it, it is generally not recommended to use out of date software no matter what. Now, I don’t know if you consider an app that hasn’t been updated in more than a year outdated but that would I guess depend on your threat model.
I personally would use an alternative encrypted note taking app as anything else would be better than what you’re currently using but this is a personal opinion.
You can still use Notesnook and disable sync and/or not provide the app access to internet and use it local first encrypted app. Much better way to go about it.
Keepass isn’t a cloud based PW manager either, but if you need access to you’re passwords on other devices, I suggest using Syncthing for your Keepass database. That’s what I do and it works great.