Hi @brett, the tag accepts the contents just fine. Check the screenshots below.
For this example, I used the Universal Conversions Variable template that picks a GA4 items array and transforms into a FB CAPI contents array.
contents in the outgoing request to Facebook.
Please, share more details and screenshots from your setup.
Are you performing some sort of external request to enrich the items before transforming into contents? If so, this can explain why the tag preview shows undefined while Variable tab shows the expected value.