I experimented a bug in the user interface.
I entered a transaction in my account register. After I clicked on save, the new transaction was not displayed. Instead, a new empty transaction line at the end of the register was displayed.
I though I missentered it, and retried. I entered it again and the same thing append. A new empty line was displayed beside the previous one.
I changed to the budget view and came back to the register view with no success.
I restart YNAB4 and my two transactions were correctly displayed.
(This bug was present in YNAB 3 but a view change fixed the display)