Impacted versions:

At least 8.0

Steps to reproduce:

See https://youtu.be/kmXVzf3ApaM

Current behavior:

Currently if you validate an invoice (even with an invoice date) after the initial conversion rate has changed
(if the rate is changed between creation and validation). The tax amount and the credit/debit amount is not computed from the same rate.

This one is responsible for several production issues for tax reporting for the fiscal administration.

Expected behavior:

The tax amount and the credit/debit amount must be computed from the same rate.

Video/Screenshot link (optional):

https://youtu.be/kmXVzf3ApaM

Related issue : https://github.com/odoo/odoo/issues/13473

My proposal to fix this one : https://github.com/odoo/odoo/pull/13621

addons/account/account_invoice.py
readonly=True, states={'draft': [('readonly', False)]})
    origin = fields.Char(string='Source Document',
        help="Reference of the document that produced this invoice.",
        readonly=True, states={'draft': [('readonly', False)]})
    supplier_invoice_number = fields.Char(string='Supplier Invoice Number',
        help="The reference of this invoice as provided by the supplier.",
        readonly=True, states={'draft': [('readonly', False)]})
    type = fields.Selection([
            ('out_invoice','Customer Invoice'),
            ('in_invoice','Supplier Invoice'),
            ('out_refund','Customer Refund'),
            ('in_refund','Supplier Refund'),
        ], string='Type', readonly=True, index=True, change_default=True,
        default=lambda self: self._context.get('type', 'out_invoice'),
        track_visibility='always')
    number = fields.Char(related='move_id.name', store=True, readonly=True, copy=False)
    internal_number = fields.Char(string='Invoice Number', readonly=True,
        default=False, copy=False,
        help="Unique number of the invoice, computed automatically when the invoice is created.")
    reference = fields.Char(string='Invoice Reference',
        help="The partner reference of this invoice.")
    reference_type = fields.Selection('_get_reference_type', string='Payment Reference',
        required=True, readonly=True, states={'draft': [('readonly', False)]},
        default='none')
    comment = fields.Text('Additional Information')
    state = fields.Selection([
            ('draft','Draft'),
            ('proforma','Pro-forma'),
            ('proforma2','Pro-forma'),
            ('open','Open'),