Previously, the structure blocks displayed a preview of custom blocks. In this new version, they do not, which makes it difficult to construct custom structures.

To reproduce this issue, create a world with any addon installed and try to use a custom block in the area of a structure block. You will notice that most of the custom blocks (not all) are invisible in the structure block interface.

Expected Results:

Custom blocks should be displayed in the structure block preview.

Actual Results:

The blocks are not displayed, or they appear as invisible in the structure block interface.