By the looks of it, your formula returns a list of dates (even though it only contains a single date). Try adding .First() at the end and it should work.
See this
Hello Everyone,
Today we are launching another exciting improvement to make reading and writing formulas in Coda a little bit easier.
We have heard a lot of great feedback from our community that it can be hard to tell why a formula is not working as expected. Two of the most common issues we see are the result of comparing or using values of the wrong data type and picking objects from the wrong table.
We hope to improve these situations and many more with the introduction of formula chips.
…
And this
Not sure if this is what you’re getting at, but it seems similar, anyway:
Manually entered tasks with client selected from drop-down list = single row.
“Current client” field returns a list of some kind (e.g., Filter(), Lookup(), etc).
Tasks created by button copy the list-of-one value to the client field, which looks identical to a single row value, but is actually a different object (list vs. row).
Hence the two groups, because they are actually two different objects, even though they don’t …