buy x get y incorrect TAX

I've defined a coupon buy x get y.
I want to get one Y product with (TAX 21%) for free when I buy an X product (with tax 10%).
In the cart I get

X product price 37.27 TAX(10%) 3.73 TOTAL 41.00
Y product price  8.07   TAX(21%) 1.69   TOTAL 9.76

       Total products                                                  50.76
       coupon free product Y                                     -9.76
      TOTAL                    TAX            4.37               41.00
                                      (TAX10%)   3.01
                                      (TAX21%)   1.36

The total tax is incorrect, if I have defined 100% discount over Y product  the TAX 21% should be 0.
So the Total should be  the same as if I only bought X product.

Please how can I solve this issue? It's very important.

Thanks,
Jordi

Do you have discount before tax setup in configuration?

I've tried it but the result is the same, the only thig that change is in the coupon line print also the negative TAX, but the Totals are the same.
X product price 37.27 TAX(10%) 3.73 TOTAL 41.00
Y product price  8.07   TAX(21%) 1.69   TOTAL 9.76

       Total products                                                  50.76
       coupon free product Y            -1.69               -9.76

      TOTAL                    TAX            4.37               41.00
                                      (TAX10%)   3.01
                                      (TAX21%)   1.36

Is this a Virtuemart site?  If so, how is the tax setup?

Yes is virtuemart.
There are tow taxes 21% and 10% each product aply only one tax. Product category select the tax to apply.
The tax setup for 21% VAT is this (the same setup for 10% VAT).

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The latest version 3.5.5.3 should fix this.

This is fixed in the latest version, 3.5.5.4.