Ordering result by custom field

Hi,

We use ASP to list and filter projects on https://rail-research.europa.eu/rail-projects.

A project is a CPT that has custom fields, all implemented with ACF Pro.

We have a custom field called project_type which is a select list with the options:

1 : Flagship
2 : Exploratory research

We want to display the projects ordered by project_type with “Flagship” projects first.

In the index we configured ordering (Search sources > Ordering):
Primary ordering = Custom Field ascending
Field name = project_type
Field type = numeric

But, when we access https://rail-research.europa.eu/rail-projects it is ordered by the default publication date.

We tried also:
Primary ordering = Custom Field descending
Field name = project_type
Field type = string or date

But it didn’t work.

However, when refreshing the page the projects are ordered according to project type like we want.

How can we have the order we want on the initial page load?

Thanks.

PS
Unfortunately we cannot give you access to the backend, we don’t have permission from our client.