Short answer: macOS Ventura 13.7.6 is not fully compatible with the Notes app on iOS 18.5, especially when it comes to viewing or editing notes that use features introduced in iOS 17 or 18.
A bit longer answer: Apple quietly upgrades the Notes app in ways that aren’t always backward-compatible. If you’ve created or edited a note on iOS 18.5 and it uses newer formatting, inline PDFs, rich content types, or even collaborative tools introduced post-iOS 17, those notes may not open on Ventura at all. Unfortunately, this isn’t something you can fix without either upgrading your Mac or reverting to an older iOS version—which isn’t possible in most cases unless you have saved SHSH blobs or are jailbroken.
So, which device should you upgrade first?
Since your Mac is from 2017 and maxes out at Ventura, it will never be able to natively run macOS Sequoia or the upcoming Tahoe. That makes your Mac the limiting factor if you want full feature parity across your Apple devices, especially with Notes and iCloud syncing.
Your iPhone 11 Pro, while aging, still runs iOS 18 and is likely supported through iOS 19. It’s slower than newer models, sure, but it can still run current apps and features, including most of the iCloud ecosystem. If you’re trying to prioritize one upgrade, I’d say replace the Mac first, preferably with a model that supports macOS Sequoia (any M1/M2/M3 MacBook will do nicely). That’ll immediately resolve the Notes compatibility issue and give you a machine that’ll stay current for years.