Hi Stephen,
Sorry for the inconvenience caused.I was able to reproduce the issue here and have created a defect report regarding this.We will try to fix this issue as soon as possible.
You could also track the progress of this bug fix using this link
Defect #1681 - BorderColor property of DockingManager does not painting entire control''s border.
I will inform you, if any workaround have been made by us.
Thanks for using Syncfusion products.
Regards,
Joy