I have already create a ticket for this problem, the answer is crazy
After an internal discussion, with the product owner and the development team, this is the standard behavior. The fact that, when there are no transaction lines on the bank journal, odoo does not display the balance in GL on the bank journal. And/or when there are journal entries on the bank account linked to the journal, they do not add them to the value displayed on the journal in the "Balance in GL" field.

Indeed, since the performance of the dashboard on database was becoming catastrophic. Especially because of this Balance in GL which has to recalculate transactions from the beginning (so you have to screen all database items every time), it was completely overkill.

This is why they changed it. Since today, it just screens the bank transactions since the last known statement and it adds these transactions to the Ending Balance of the last known statement.