Forge version 1.16.5 / 36.2.2
IE version 1.16.5-5.0.3-138
IP version 1.16.5-3.3.0-5

When placing multiblocks with the projector while in creative, if you mirror the multiblock, the placed blocks don't have mirrored blockstates, but their blockstates need to be mirrored for the structure to form with the hammer.

One examples would be the Coker. When mirrored and placed, fences and some other blocks are connecting the wrong way. Another would be the Bottler, which requires it's conveyors to be rotated the other way before the block will form. Out of the two multiblocks I tested, these were the only ones that wouldn't form when mirrored, but there may be others.

There may be an image below. If so, it probably shows the back of two cokers. One is mirrored. You can see the blockstate issues in the mirrored one.