Minecraft 17w49b, Minecraft 17w50a, Minecraft 18w01a, Minecraft 18w02a, Minecraft 18w03b, Minecraft 18w05a, Minecraft 18w06a, Minecraft 18w07c, Minecraft 18w08a, Minecraft 18w09a, Minecraft 18w11a, Minecraft 18w14a, Minecraft 18w14b, Minecraft 18w15a, Minecraft 18w21b
No error message appears in the chat or game log when attempting to save a structure that contains uppercase characters. The structure file also doesn't appear in the "generated" folder if that's the case.
To reproduce place down a structure block, switch it to SAVE mode, input a name containing an uppercase character, click "Done". Come back to the GUI to press "Save". Notice no error message appearing and no structure file being generated.
This leads to confusion and (probably) bug reports similar to