- Create a move
- Add a line and set a custom tax tag on it
- Add a new line to balance the move
=> The tax tag is gone.

This is because the code is recomputing taxes when adding a new line.

closes odoo/odoo#135756

Opw: 3487465
Signed-off-by: Brice Bartoletti (bib) <bib@odoo.com>