The Save Structure Blocks dont save anything after you detect a structure.
Comments 2
Confirmed for 18w22c.
When attempting to save structures, it no longer works, unless you save twice. This may have to do with how it updates the name to minecraft:<structure name>, which it didn't do before.
Also of note: when saving structures using relative position (usually detected using corner blocks), without the accompanying corner block, it is unable to detect anything. This wasn't the case before, as I have been able to save large structures without corner blocks.
What is the name of the structure you are trying to save? Does MC-123064 describe your issue?