Unfortunately that is an issue with the input mask in Mendix. If the first character of the mask is not variable it behaves kind of strange.
In your case your first character is a ( this is giving you this problem. If you would remove the ( it behaves much better.
Please enter a ticket for this if you think this should behave different.
is there a solution here in the meantime? I currently have the same problem. The first three characters of my string are static.
I want that the cursor always start at position 4, no matter where I click into the textbox.