The bug

When you save a structure only temporary by activating a SAVE structure block with redstone instead of pressing the "Save" button and then power a CORNER structure block with the same structure name, the structure is deleted.

Even if this is the intended behaviour, this is not documented anywhere and is pretty counterintuitive. There should be a proper way of doing that, for example a DELETE mode.

How to reproduce

1. Place a SAVE structure block with the structure name "test"
2. Activate it using redstone
3. Place a LOAD structure block and load the structure called "test" to make sure it works
4. Place a CORNER structure block with the structure name "test" and power it
5. Place a LOAD structure block and try to load the structure called "test"
   → It says that the structure is unavailable

The reason

The following is based on a decompiled version of Minecraft 1.10 using MCP 9.30.

The method net.minecraft.block.BlockStructure.func_189874_a(TileEntityStructure) calls a method to remove the structure from memory if the mode is CORNER.

Note: This affects structure block files as well, but for them this has not a really big impact because the game just loads it into memory again the next time it is referenced.