pg_upgrade from 9.4 to 10.4

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.