We had same issue after move to 7.3.
I had narrowed it down to the fact that the generated authorizations is a 'Node 'authorization, and what's missing was then a 'Page' authorization:
It seemed to be only valid when the authorized hierarchy node has other nodes with validity periods ended in the past, so this way making the authorized node not a 'Node' anymore but rather a 'Leaf/Page'.
Had an OSS message open on the topic for half-year with only answer "works as designed".
The solution, however, is to generate authorizations twice - once with 'InfoObject Node' mapped, and one more time without the mapping..