Notes on mac has disabled undo immediately after a type-over event

Can't find any other mention of this issue anywhere. Today I was typing in a note, accidentally hit ⌘+A and continued typing, which means the entire note was selected, and all the text was replaced with a couple characters (from the end of the word I was typing).

No big deal, definitely not the first time it's happened (or the last). I hit ⌘+Z to undo it, and got the error-bump sound. No undo. Went to the edit menu and "undo" was grayed out. Never had that happen. Usually Notes (or any app at all, in 2021) will let me undo until my fingers bleed, much less the last 3 character strokes.

It wouldn't be a big deal, but this is my main note. Years of to-do lists, things that I'll never recreate from memory. I can't lose this note without understanding wtf is going on, otherwise I need to leave the cloud and start self-hosting again. Auto-save without undo is a very bad setup. I can't even think of a way to back this stuff up, a restored MacOS state would just pull the iCloud data as soon as I open Notes.

After the error I didn't touch the app, didn't want to upset the state records (however those are stored). Talked to lvl1 phone support, they seemed to have no idea what to do. They seemed prepared to help me recover a deleted note, but not a state loss.

A senior advisor suggested that if I "saved" the note after the typing error, the old states might have been discarded. I was confused and told her that Notes via iCloud doesn't have a save command, afaik. She asked if I could see the old note state on another device or icloud.com, which obviously I can't... live updating across devices is the entire point of icloud. If the old state persisted on one of my other devices that would be a very lucky bug to experience at the same time as an undo bug.

I couldn't undo the mistake on my other devices, which I expected bc I did the typing on my mac. She also suggested that since icloud live-updates, old states are gone with each new input. I was starting to get the impression they knew less about undo in iCloud than I do (which is not much), so I gave up and came here.

Are undo states stored somewhere locally? Or anyone had luck getting Apple to pull them on their end? What might cause the undo states to reset like this, as if no changes occurred before the typing error?

Big Sur 11.2.3 (20D91)

MBP 16 - 2019

Notes 4.8 (1821)