Text field value is correct only when cursor is inside text field. (Custom Calculation Script)

I use Acrobat Pro DC 18

I have a text field that show the last 3 digits of a select field value (using custom script).

The problem is that it only shows up the 3 digits when the cursor of the mouse is inside the text field. When it's outside, the value is not updated.

If I changed the select box value, the value is correct in the text field.

I fill out the pdf with PDFTK, I added this javascript when opening document so the calculations are triggered :

this.calculateNow();this.calculateNow();

Other fields work just fine. In another field I sum up this value and other fields value and the total is correct. Only the display is not correct for this field.

1. Cursor outside : value is not correct

2. Cursor inside : value is correct

3. Cursor outside : value is not correct

4. Changing value : value is correct :

It works on google chrome pdf viewer. :

Why the display value is not the same as its value ?