You are able to place blocks inside of a structure void in creative survival and adventure mode.
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However, this issue is Working as Intended.
The report you have submitted is working as intended: MC-102908.
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I think this works as intended, the point of the structure void block is to make structure blocks ignore whatever block is there. (Structure voids can be placed as part of a structure and are ignored when the structure file is saved. This results in the blocks at the location where the structure is placed being maintained and not being overridden. For example, if a player saves a 2 block high structure with air at the top and a structure void at the bottom, and loads it into a 2 block high area of stone, the top block becomes air but the bottom stays stone.) (Source: Minecraft Wiki)