I’m not actually certain if this is a file type limitation or a Vectornator bug, but layers/layer groups with opacity and/or blend modes don’t export correcty for Vectornator on iPad.
To counter I currently write down their opacity/blend attributes, revert them to Normal 100%, export as a separate SVG, import that and the other parts of an image as two layers in say Photoshop on desktop, and re-apply the blend/opacity to the relevant layer and save from there.