I don't know but I'm interested to find out. Have you seen any insects on the leaves or around the plant at all? I'm tempted to say it's bug damage due to the puncturing nature of the damage. There is a specific sedum bug but it was quite rare until it started spreading recently so there's not much information out there about it. https://www.britishbugs.org.uk/heteroptera/Miridae/chlamydatus_evanescens.html
If you can keep your head, while those around you are losing theirs, you may not have grasped the seriousness of the situation.
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