This is a classic Advantage+ pattern. It finds a hot pocket of buyers straight away, giving you that great initial ROAS. Once that small group has been exhausted, the system has to go broader to find new people, which almost always causes a performance dip.
Your main suspect should be creative fatigue. The initial audience has seen your ad too many times. The single best thing you can do is introduce some fresh ad creative to the campaign. For instance, I remember one campaign we ran for a women's apparel brand that drove a 691% return on ad spend, and a huge part of maintaining that performance was continually testing new ad creatives.
Also, have a quick look at your metrics. Has your click-through rate dropped off a cliff? If so, it's definitely the ad. If clicks are still coming but your website conversion rate is what's dropped, then the broader audience Meta is now finding isn't a good fit for your product or pricing.
Hope this helps.
