Date unit buckets missing on custom date field

Hi there!

I'm on version 50.31 and i'm struggling to create a custom date field that allows me to group by using its date buckets (minute,hour,day...), so I would like to know if thats the normal behaviour or if i'm doing something wrong. Here's what I'm doing:

1- I've created a question querying a mysql table, adding a custom date column by extracting and evaluating data from other fields.
2- I've turned that question into a model
3- I've set the metadata of the custom date field to "Creation date"
4- I'm trying to query that model, but when grouping by that field, it doens't show the bucket options like other date fields
image

Hope somebody can point me on the right direction about this.

Thanks in advance!