This is what happened on my system in standalone SpectraLayers, only it got stuck at something like 3%. Robin Lobel posted a test patch in the SpectraLayers forum, and that resolved the issue for me (again, in standalone – I haven’t attempted SpectraLayers in Cubase recently). Here’s a link to that post:
The explanation give was, “It limits VRAM use to dedicated VRAM, unless you explicitly check “Use Shared Memory” in.” Once I tried that on the same song as had hung on me with the released version (12.0.30), SpectraLayers used my graphics card (GeForce RTX 3060 with 12GB VRAM) and umixed the song in near-real-time.