Following sequence does not work as expected:
1. first tx (and first session)
make it lockable
2. second tx (and second session)
lock this node and save lock token
3. third tx (and third session)
add saved lock token to session
modify this locked node -> fails as if lock token was not added to session3
The same sequence works as expected without transactions.
I had to separate transactions 1 and 2 because
JCR-1633 prevents node from being locked in same tx in which it was created.