I don’t know if it was done on purpose or not.
Invoice taxes (lower-left corner of the invoice form) can be computed automatically from the content of invoice lines and the user can change the computed amount without problems.
It is also possible to create new tax lines manually. In this case, if the user selects a tax, it is possible to define the base of the tax but it is not possible to change the amount. If amount is changed by the user, Tryton automatically puts the computed tax again.
This is due to this on_change_with method:
Is this on purpose?
We’ve got some cases in which it would be useful to add manual taxes but we need to be able to change the amount manually too.