This was noted back in 2015 I see (Why are taxes calculated incorrectly when "other amounts" enabled on contribution page?). It doesn't seem like it would be hard to fix, but here we are almost 5 years later and the problem persists. It works in a price set, so there is a workaround, but it's a little uglier there when used for simple scenarios.
In this case, the tax rate is 13%. Calculated properly if fixed options are provided below, but calculated on 1 if the "allow other amounts" is checked and the box is used. I assume it is using the same logic that it does in a price set where it sets the value to $1 and then uses the box as a quantity. However, the tax calculation step is seemingly performed too soon. Any chance it could be fixed? Happy to take a pointer to the related file(s) to have a look for myself.