In ads manager while setting up the ad you can choose which placements you want the ad to use. If you click “automatic placements” Facebook will check which platform / placement will work best for your post and deliver your ad primarily there.
Then if you set up the ad and choose your pre-existing Facebook post, Facebook will create a sort of mirror image of that post on Instagram. If this mirror image performs better than its original on Facebook, the algorithm is going to allocate most of your money to it.
You can do two things to take control over this mechanism:
(1) You could just use manual placement instead of automatic placements and specifically choose where you want the ad to be displayed.
(2) If you still want to profit from the better performance on Instagram, you could use campaign overarching budget instead of target group budget (not sure if this is how it’s called, since I’m using the ads manager in another language). When setting up the ad, right after choosing the ad objective, you can activate the option to have an overarching budget for your campaign that will automatically be allocated to your targeting groups. Then you set up two different targeting groups for the campaign and choose Instagram as placement for one and Facebook as placement for the other. In the ad segment of the campaign you can now choose the existing Instagram post as your ad for the Instagram targeting group and vice versa for the Facebook targeting group. That way there will be no new ad created and all the engagement will be shown on your profile.
I hope I made it clear enough, I’m not sure about some of the vocab, since I’m doing all this in my native language. If you have any follow up questions, feel free to hit me up.
Edit: In your target groups you can even set a minimal budget that should at least be spent to make sure it’s not all getting blown into Insta again. That’s right where you set the ad’s running time. You have to click something like “advanced options” to be able to change this.
Another Edit: If you set up the ovarching campaign budget, make sure that you don’t use daily budget, if you don’t want to. It can be easily mistaken und you can spend a lot more money than you want, if you don’t keep that in mind.