Both vanilla and custom structures are affected by this, and will have to be updated manually.
The vanilla stuctures getting updated is being worked on, for that see MC-122188.
structure blocks are not saving my build on 17w47a + 17w47b
Mansions do not spawn properly
Structure block not loading structure from 1.12 in 18w11a
Structure Blocks Don't Save, nor Load files to/from computer in the new 1.13 snapshots
Structure block not loading player made structures created before 1.13
1.13 doesn't convert old structures correctly
Vanilla structure files aren’t updated yet causing missing/wrong blocks