Does Postgres lock all rows in a query atomically, even across different tables via JOIN?

Question

I am getting a deadlock error on my code. The issue is that this deadlock error is happening on the very first query of the transaction. This query joins two tables, TableA and TableB and should lock a single row in TableA with id==table_a_id, and all the rows on TableB that have a foreign key for table_a_id.

The query looks as follows (I am using SQLAlchemy, this output is from printing the equivalent query from it and will have its code below as well):

SELECT TableB.id AS TableB_id
FROM TableA JOIN TableB ON TableA.id = TableB.table_a_id 
WHERE TableB.id = %(id_1)s FOR UPDATE

The query looks as follows in SQLAlchemy syntax:

query = (
    database.query(TableB.id)
    .select_from(TableA)
    .filter_by(id=table_a_id)
    .join((TableB, TableA.id == TableB.table_a_id))
    .with_for_update()
)

return query.all()

My question is, will this query atomically lock all those rows from both tables? If so, why would I get a deadlock already exactly on this query, given it's the first query of the transaction?