Your group is obviously a multivalue attribute.
I.e. this is a list.
I think you need to use the comparator contains (See 7.9. Policy conditions — privacyIDEA 3.6.2 documentation)
But if you do not post your configuration, your policy with the condition and the resolver configuration it is all poking in the dark.
Please read Is privacyIDEA an open source or a commercial solution