This is a very annoying bug because you hold shift when you want to capitalize a letter. It's not the fact that you can't capitalize anything when writing in a structure block. It's only that it won't type when shift is being pressed down.
HOW TO RECREATE
Open a structure block, press shift while typing, realize you have to type that letter again.
(I don't have a way to show
Duplicates MC-135897.
This behavior is Working as Intended.