I'm currently having a mod in development, and i built a little structure for it with a modded blocks. I exported it with MCEdit 2.0.0-beta12 into .schematic file, imported it into MCreator, found it and it was all made of Gold Ore and Oak Leaves blocks. This is not the problem with MCEdit - i imported the same .schematic file in the world and everything was fine. I also attached a few screenshots with that structure - the original, the copy, and the generated structure.
Issue comments
MCreator 1.8.0 will support structures by MInecraft and custom blocks in them. I am closing this ticket as this will be fixed in 1.8.0. Check the changelog to learn more about all changes related to structures: https://mcreator.net/changelog
A similar bug was described here: https://mcreator.net/tracker/issue/43069
It is known that custom blocks in schematics don't work directly with MCreator. We will improve this in future updates. I will keep this ticket open as it is not totally the same bug as in the ticket linked above.