ok, fixed in 5.30. But the fix can be applied to 5.29 if you like.
In NetWeb.Tpw (\clarion6\3rdparty\template or \clarion8\accessory\template\win)
lines 11500 and 11557
change
#Set(%vl,'''mailto:''&'&%nFieldEmail)
to
#Set(%vl,'''mailto:''&'&%nField)
Sorry for the bug, and thanks for the report.
Cheers
Bruce