Re: ERROR: wrong varnullingrels (b 5 7) (expected (b)) for Var 3/3

Thanks for the test case, issue reproduced here on HEAD and not v15.
This causes an assertion failure here:
#4  0x000055a6f8faa776 in ExceptionalCondition (conditionName=0x55a6f915ac60 "bms_equal(rel->relids, root->all_query_rels)", fileName=0x55a6f915ac3d "allpaths.c",
    lineNumber=234) at assert.c:66
#5  0x000055a6f8c55b6d in make_one_rel (root=0x55a6fa814ea8, joinlist=0x55a6fa83f758) at allpaths.c:234
#6  0x000055a6f8c95c45 in query_planner (root=0x55a6fa814ea8, qp_callback=0x55a6f8c9c373 <standard_qp_callback>, qp_extra=0x7ffc98138570) at planmain.c:278
#7  0x000055a6f8c9860a in grouping_planner (root=0x55a6fa814ea8, tuple_fraction=0) at planner.c:1493

I am adding an open item.