If you're using a multi-treatment output feed in Meta then you have the ability to assign more than one treatment to an ad in order to support aspect ratios that work better for different placements. Before or after assigning the treatments you will need to make sure that the "Adapt to Placement" enhancement is turned on at the ad level in Meta.
Be advised that the 4:5 treatments may not be visible in the ads manager preview. To our best knowledge they should still be visible when the ad is served to the user.
Different placement sizes of the same creative must be placed into their own treatment slots. 4:5 and 9:16 placement sizes are not able to be assigned without one treatment to assigned as the main treatment (we recommend 1:1 in most cases).
From the output feed details page click assign ads.
Choose the campaign and ad set you're looking to assign.
Click the ad you're looking for to open it's details.
Select a treatment to assign as the main treatment for the ad.
Now you will be able to assign a 4:5 and 9:16 treatment to the ad. You can assign one or both if you desire.
Save the changes as normal.
🤔 Keep in mind that these are all separate treatments even though they are attached to the same ad. To support multiple aspect ratios you are adding more images into the catalog which will counts towards the 20 image/SKU limit for a Meta catalog.