When I subdivide, I can delete the lower subdivision and reverse without a problem. But when I save the file and re-open and try to reverse subdivision it throws up the “probably not a subdivision mesh” error.
Keep triangles is unchecked.
It seems to only happen with this particular mesh. Testing on a different project works fine.
Maybe it’s a combination of dynamic topology and global remesh (with some areas masked) that’s making the topology difficult?
You can send me the file and I can take a look.
Is it an old project?
It can fail in some rare cases (degenerates faces, duplicate vertices that has been merged on loading, etc)
As far as I know any voxel, global, dynamic, it will remove the subdivision. Since you delete it, the system assuming you don’t need it, so if add voxel, global, dynamic it will literally flush the history.
You will get the warning if you don’t delete it. 
File incoming.
Project I started about a month ago. Saving iteratively.
@Dface_Lesh , I subdivided once as the last step in the process. So i didn’t alter it all afterwards.
Thanks.
Well I guess it a bug. 