Inserted some parts into a top level assembly and mated them to a bracket. Then created a sub-assembly and moved the parts into the sub- assembly. The bracket was left at the top level.
The bracket needed to be modified (hole spacing change), which caused the top level mates between the bracket and the parts in the sub- assembly to fail.
The only solutions I've found are to dissolve the sub-assembly and re- create it, or enable the flexible sub-assembly option. Making the sub- assembly flexible resolves the mate errors, but does not display the parts in their correct positions when creating a detailed drawing of the sub-assembly by itself.