Attempting to load structures with entities doesn't work in 21w11a, even if the Include Entities setting is enabled.
The attached screenshot contains the structure I was trying to save on the right, and the result of loading it on the left.
MC-220352
Structure blocks don't save entities
Resolved
Invalid
0
Please close this report. I just realised that I've been a complete idiot and missed the fact that there is an Include Entities button on the Save and Load menus.