This bug took a lot of hours to track down - I found 2-3 unrelated bugs on the way and various dead end rabbit holes before realising what was happening.
Twig has traditionally used output buffering to render templates. This is incompatible with Fibers, because once the output buffer starts, other things can be outputted to it out of order.
This has actually been fixed in Twig already, see @fabpot's note in https://github.com/twigphp/Twig/issues/3599. However, Twig still uses output buffering by default, yield is only an option. So we need a two line change to core, then it should work.
I will post a fix-only MR here soon, this can then be integrated into 📌 Entity lazy multiple front loading Active and give us a green run there.
I tried to write a failing kernel test for this earlier in the week, but failed - however that was before I understood the root cause of the bug so maybe I can adapt that to expose the issue now. Will add that later.
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