Dynamic fields not working with Gantt visualization

[rjromay]

I'm using version 9.7.1 (aws elasticsearch domain with service software release number R20210331).

I tried to use a Gantt visualization using a dynamic field for the duration as:

I can see the field values in Discover tab, the diference in milliseconds, and I can also use the field in other visualizations.

But when I use it for the Duration field in Gantt visualization, the bars aren't shown.

If I use any other number field in the index, the bars are shown starting on the correct time:

I created then another Dynamic field concatenating two other string fields:

Again this field can be used everywhere but when used in the Gantt visualization: