To some extent, this always happens on all advertising platforms, because when you increase your budget, that platform wants to sell you more clicks to use up your whole budget and they show your ads to people that may not be ideal audience based on their internal metrics.
Let’s say your targeting parameters match to an audience size of 1 million people but you set a small budget and your ad is shown only to 10k people per day. The platform can decide which of those 1 million people are going to see your ad and they tend to show it more to people that have higher engagement with ads. They wanted them to click your ads a lot because you are paying for clicks (I assume).
You can only have so many "responsive" users within the audience. When ads get displayed to 2nd, 3rd tier traffic quality and so on, CTR drops, their internal CPM goes up and now they want to charge you more for those clicks because they they are no longer earning as much from getting you clicks as they did in the beginning with the initial micro audience that responded well and clicked ads a lot.
If your CTR is good and your ad platform allows for CPM payment model, you could try switching your best ads to that and see if it helps. Good luck.
