A simple structure like:

join
  access 
  union
    access
    access

will reject creating a cost based dependent join over the union if the number of independent values seems too large for a virtual dependent join.

A related issue is that TEIID-5020 introduced a regression when creating a dependent join that is eventually pushed over a union below an access node:

dependent select
   access
      set op
        ..

the logic attempts to move nested dependent select back above the access node - so that only a single dependent predicate exists - based upon the node that is created at a child. However if the child projects a literal into the dependent set criteria, then the result is invalid.