Reproduced in 1.21 (version/1.21 from source). Did NOT occur off the trunk (version/main).

Pick the dynamic frame and make a 2x2 block (I used Oak Log over Oak Plank).
Scan it.
Paste it as a schematic.

Textures change (original left, pasted right).

Same steps off trunk worked fine.

No mods installed. (*Edit no mods other than Minecolonies required mods...)

This should work fine with minecolonies installed, just not with only structurize.

Ohh, 1.21

Ah sorry, I was unclear - I tested through the minecolonies version/1.21 source code branch. So structurize, domum, minecolonies, blockui were all present.