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
else:
            taxes = product.supplier_taxes_id or account.tax_ids
            if product.description_purchase:
                values['name'] += '\n' + product.description_purchase
        fp_taxes = fpos.map_tax(taxes)
        values['invoice_line_tax_id'] = fp_taxes.ids
        if type in ('in_invoice', 'in_refund'):
            if price_unit and price_unit != product.standard_price:
                values['price_unit'] = price_unit
            else:
                values['price_unit'] = self.env['account.tax']._fix_tax_included_price(product.standard_price, taxes, fp_taxes.ids)
        else:
            values['price_unit'] = self.env['account.tax']._fix_tax_included_price(product.lst_price, taxes, fp_taxes.ids)
        values['uos_id'] = product.uom_id.id
        if uom_id:
            uom = self.env['product.uom'].browse(uom_id)
            if product.uom_id.category_id.id == uom.category_id.id:
                values['uos_id'] = uom_id
        domain = {'uos_id': [('category_id', '=', product.uom_id.category_id.id)]}
        company = self.env['res.company'].browse(company_id)
        currency = self.env['res.currency'].browse(currency_id)
        if company and currency:
            if company.currency_id != currency:
                values['price_unit'] = values['price_unit'] * currency.rate
            if values['uos_id'] and values['uos_id'] != product.uom_id.id:
                values['price_unit'] = self.env['product.uom']._compute_price(
                    product.uom_id.id, values['price_unit'], values['uos_id'])
        return {'value': values, 'domain': domain}
    @api.multi
    def uos_id_change(self, product, uom, qty=0, name='', type='out_invoice', partner_id=False,