A Virtual Column set to TYPE=Text always returns NaN

[Eso_Surveyors]

hi,
looks like this:

formula is this:
LOOKUP([STAFF MEMBER], “STAFF LIST”, “E-Mail Address”, “DATE START”)
where DATE START is concatenated text from another Date column in STAFF LIST
but it always displays like this:

when (deleted and) created TYPE is always Number and I change to Text.

I have other APPS with VC’s that calculate from columns in the same table they are in,
but this time i need to read from another table in the APP, is this the reason it fails?

cheers,
Mike