Sorry I didn't realise this was conceptually difficult. Continuity markup works as advertised PRECISELY ONCE. I have tested this with two Macs, one iPad, two iPhones, two AppleIDs, constant Wi-Fi, constant Bluetooth, all updates (to Monterey and iOS / iPadOS 15). I can markup a pdf ONCE via an iDevice and view the subsequent "drawing" annotation in Preview – and indeed share it via SharePoint with Windows users. What I can not do is open that same pdf and add a SECOND markup from an iDevice without losing the first, from all versions of the pdf viewable via any device. Is that just me, or is that a feature? Please don't reply with troubleshooting 101 unless you have tested otherwise.
By the way, there is no concept of "opening" and "closing" the pdf "on the iPad" when doing this. You click a button to push it to the iPad, edit and hit "Done". Do it again and what you did before vanishes, regardless of whether you exit QuickLook on the Mac…