Ah, makes sence.
Well the ultimate objective is in line with this topic. So indeed…
Since there is no way to union two collection fields using a formule we have created a workaround like Yvette explained in the other post.
But we also need to be able to make corrections, when a lead isn’t interested in a product anymore. So to make it happen we
In our case I can’t see how an automation could update the [Instant - Product ID’s], because it is a join formula based on the linked product ID’s.
If(
[Uitsluiten van 'Interesse in'] = true,
"",
If(
[Interesse in product].Count() < 1,
"",
[Interesse in product].Join("[" + [Public Id] + "]", " ")
)
)
Think we need to find a different solution (read workaround). 
