Creating or recreating an index is not logged to MON$STATEMENTS, so you see an active attachment and and active write transaction but you don't know what it is doing.
We have a service checking for long opened write transactions and close the associated attachment after some minutes.
This is for prevent problems in our server when some developers open a query in a developing tool and then go to lunch forgetting to commit it.
But sometime the transaction is valid, for example when it is creating an index on a large table.
Due to this issue we deactivate the service because there is no way to detect what is doing the transaction.