Hi Darren,
Thanks for your interest with Syncfusion products.
When using the Border style as Fixed3D we must set the ThemesEnabled property as False in DoubleTextBox.By default the value is true.
I have prepared the simple sample and here I have attached it for your reference.
[Sample Link]
DoubleTextBox883486497.zipPlease have a look at the above sample please let us know if you have any concerns.
Regards,
Rajasekar R.