The reason for the 25 collections limitation is related to a Facebook limitation on the ad set level.
The creation AI module is setting 1 Facebook ad set per AdScale Master Ad, to optimize the performance and audience targeting optimization on Facebook.
Facebook limits the number of ads per ad set. Therefore, choosing more than 25 collections can bring the situation that some products and collections will not be included in your Facebook campaigns. To prevent that, we limit the number of collections you can choose per campaign.
In case you would like to target more than 25 collections, we recommend creating an additional Master Ad in the same campaign.