Jigsaw blocks do not disappear post-gen when structure is spawned using the 'Generate' action from an initial jigsaw

When spawning a structure from the start piece in a jigsaw block in already fully generated chunks (not proto), jigsaw blocks stay in the structures.

See attached screenshots on how to reproduce

https://cdn.discordapp.com/attachments/674291887409987625/700121602687500389/2020-04-15_22.27.22.png https://cdn.discordapp.com/attachments/674291887409987625/700121572509483028/2020-04-15_22.29.22.png