The field works OK but you notice that the account segments don't seem to fit correctly and are cut off.
This could be an issue with the Account Format Setup in Dynamics GP to allow for "wide" account segment values such as "W" however you know this isn't it the problem because the account number fields in the rest of Dynamics show OK.
So what could it be?
In this case, it is the Font of the field being used on this field.
By default, fields added to a window via Dexterity or Modifier default to the "System" font which is typically too large to fit into the individual segments.
The solution to this is quick and simple. Just change the Font property of the field from "System" to "Helvetica 12" which is the font that Dynamics GP uses for account numbers.
Once you make this change, your account numbers should show correctly.
Dynamics GP Developer Support