Parallel pg_restore generally assumes that the archive file is telling it the truth about data dependencies; in particular, that a TABLE DATA item naming a particular target table is loading data into exactly that table. --load-via-partition-root creates a significant probability that that assumption is wrong, at least in scenarios where the data really does get redirected into other partitions than the original one. This can result in inefficiencies (e.g., index rebuild started before a table's data is really all loaded) or outright failures (foreign keys or RLS policies applied before the data is all loaded). I suspect that deadlock failures during restore are also possible, since identify_locking_dependencies is not going to be nearly close to the truth about which operations might hold which locks.
This could possibly be fixed by changing around the dependencies shown in the archive file so that POST_DATA objects that're nominally dependent on any one of a partitioned table's members are shown as dependent on all of them. I'm not particularly eager to write that patch though. For the moment I'm inclined to just document the problem, e.g. "It's recommended that parallel restore not be used with archives generated with this option." regards, tom lane