Locked notes on my iPad are now hidden and unsupported after software update

The "latest version they are providing" depends on what model device you have. For example, at the same time iPadOS 17.4.1 was released for recent models, iPadOS 16.7.7 was released for iPads that are too old to run any release 17.x.

On your iPhone, open Settings to General > About and write down the iOS version.

Now repeat that on your iPad and write down the iPadOS version.

Are they the same? How far apart are they?

Which recent software update? Did you perhaps update another device to a new version of the system without updating iPadOS to match?