Hi Alan, I've never seen this happen and don't think there's many set neck bass kits so this could be a first this happening. The 3 kits I've built have all been bolt on necks. Obviously a bass neck has alot more tension on the neck and neck pocket than a guitar. So you say you filled the gaps created with more glue ? I'd make sure the gaps are filled with glue and give it 2 or 3 days to dry being winter. Try string it up and see what happens. Bracing with something stronger seems extreme. Hope more glue can solve the problem.
DB may also have ideas on a fix