source.mode "derived": two union sources share the label "{label}"; give each a distinct `label` or __source/__key will not distinguish them.

Warnings reference › source.derived.union.label.*

Identifier

source.derived.union.label.*

Raised by

The grid (core)

Since

1.51.0

What happened

Each union entry stamps its rows with its label, and that label is how a row's origin is told - and how its key is made unique across parents. Two entries sharing one label make their rows indistinguishable: __source no longer says which grid a row came from, and two rows with the same id from different parents collide.

The fix

Give each entry of from a distinct label.

Reference

Covered in full at the API reference.