Using lookup start and end date for Gantt blocks

Hello,
We are developing apps and I would like to use Gantt blocks to follow the development of functionalities.

functionalities are distributed in sprints which have a stat and end date.

in the functionalities table, I have a lookup with the start and end date (coming from a sprint table).
unfortunately, when I use this data, the gantt does not see the dates (even with proper formatting option of the lookup fields).
Same if I use a function field which display the lookup field.
I have to manually change a date field Start and End to see the gantt.
Any idea how to solve this ?
Thanks in advance

Keep in mind formatting is just that: It changes the presentation of the data, but not the underlying value.

Do you mean you have a formula field configured the with function {LookedUpDate}? If so, Airtable is seeing that as 1 January 2019 (or whatever the year is in the date). Presumably this is because of the way it handles array values: The lookup field is technically a single-element array of dates rather than an actual date. Try

DATETIME_PARSE({LookedUpDate}&'')

That will force the field into proper internal datetime format and make it correctly accessible to the Gantt Block.