this aged like milk
This was likely intentional but not anymore. This has been fixed as of 23w12a, where signs are editable. It seems to be an unused feature originally, but now you can edit signs. On the back too!
mandela effect for me then
They were in 1.19.3 Release Candidate 3, but that one wasn’t picked.
Can confirm in 1.21.1. Interestingly this doesn’t happen on Bedrock edition. On BE it just disappears.