pg_upgrade from 9.4 to 10.4

Hi,

After pg_upgrade, the data in Postgres is corrupted.

1. Postgres 9.4, stop db with "immediate" mode
2. Run pg_upgrade, to upgrade to Postgres 10.4
3. Start Postgres 10 and run vacuum full, got a bunch of "concurrent insert
in progress". Looks like the data is corrupted. (Loading the old data back
on Postgres 9.4 works just fine)

But if I stop the 9.4 DB with "smart" mode, the data after pg_upgrade looks
fine.

pg_upgrade doesn't stop or throw warnings if the upgraded db gets into
corrupted state.

I would like to understand why it happens so.
1. What transient state corrupts the db?
2. Is it a known issue with pg_upgrade?
3. Is there a way to get the data from pg_upgrade after "immediate" mode
stop of previous version?

Thank you.

Regards,
Vimal.