In the attached very simple project, the main window has a single RadExpander containing a single RadNumericUpDown control. The Viewmodel adds a validation rule which always sets the only property (MyValue) as failed. (I tried to attach files but your system no longer lets me!)
When the RadExpander.IsExpanded property is set to false (as in the attached code), and you then expand it, you'll see the RadNumericUpDown appears normally, without any validation error adornment.
If you change RadExpander.IsExpanded property in the xaml to true and you run, you'll see the RadNumericUpDown has the validation error adornment and shows the error tooltip.
Why? If you use either Snoop or the VS tools, you'll see that the RadNumericUpDown has the error in both cases.
I'm using 2017.3.1018.45 on Win 10 x64.