Why it happens
At a modest budget, the platform serves your ad to the slice of your audience most likely to convert first. That's why a validation budget looks so efficient. When you raise the budget sharply, the algorithm still has the same audience but far more money to spend in a day, so it pushes past that best-matching core to weaker matches, and into higher-competition auctions. More spend, worse average match, rising cost per acquisition. On Meta the first warning sign is usually rising frequency (the same people seeing your ad repeatedly).
The fix: scale sideways, not up
You relieve saturation by giving the algorithm new pockets of good people to find rather than over-mining one. Add a second ad set, a lookalike of your actual converters or a genuinely different audience, and let campaign budget test them. If you do raise a single budget, do it gradually so you don't reset the learning phase. We walk through it in why your Meta ads stop scaling when you add budget.