Ahhh @SlickSlack has solved the mystery! An update in OS Monterey has removed the workarounds that previously worked in Preview! We also have a solution for now because Apple have disabled the function in Preview, but haven’t disabled the same functions in Safari!
So, all you need to do is open the pdf in a browser and you can print to pdf from there, producing a “clean” version that isn’t locked. I’ve tried it with my pdf this morning and it works!
We win for now (until Apple get wise to this and also disable the function in browsers 
).