I don't know how their builds work or where their templates are instantiated but it's possible that the compiler doesn't see this optimization until LTO and that they don't do LTO until stable releases? It would seem strange though, I'd have thought beta would have LTO.

Yeah that would be a very weird choice. LTO itself can have bugs, or (as we speculate here) unmask existing bugs. Plus, even that aside, you'd hopefully test exactly the build that you will release before you release it.

As I wrote in another comment, this could still be the result of a freak accident and not have shown up in testing, though (but since I know nothing about the process or the code there, it could be anything).