What Meta says
In Meta's own words: “Only a significant edit causes an ad set to reenter the learning phase.” (Meta help center)
The published list
Every edit affects delivery, but only a significant edit restarts learning. Meta names six that always do: any change to targeting, any change to ad creative, any change to the optimization event, adding a new ad to the ad set, pausing the ad set for 7 days or longer, and changing bid strategy.
Three more count only if the change is big enough: budget amount, bid control or cost per result or ROAS goal amount, and the ad set spending limit. Meta's own illustration is that moving a budget from $100 to $101 is unlikely to reset anything, while $100 to $1,000 may.
Why it decides whether an account ever stabilizes
The learning phase ends after roughly 50 optimization events in the week following the last significant edit, so the clock restarts with every one of these. Adding a single new ad restarts it for the whole ad set. Teams shipping fresh creative every few days, believing they are optimizing, are resetting learning faster than it can finish, which is how an ad set stays learning limited indefinitely. The full mechanic and the fix order are in the Meta learning phase guide.
Advantage+ campaign budget changes the blast radius
With Advantage+ campaign budget, switching the campaign bid strategy or adjusting the campaign budget can push multiple ad sets back into learning rather than one. Budget redistribution between ad sets does not, an ad-set-level edit does not affect its siblings, and adding a new ad set does not reset the existing ones.
A worked example
Say an ad set launched on Monday has 35 purchases by Friday. Adding a new ad on Friday is a significant edit, so the ad set reenters learning and now needs about 50 results in the week after Friday; the 35 it already banked no longer count toward exit. Had the change instead been a budget move from $100 to $101, Meta says that is unlikely to reset learning at all.
Common questions
What counts as a significant edit on Meta?
Any change to targeting, ad creative or the optimization event, adding a new ad to the ad set, pausing the ad set for 7 days or longer, and changing bid strategy. Budget, bid control and spending limit changes count only if the change is large.
Does changing the budget reset the learning phase?
Only if the change is big enough. Meta's example is that $100 to $101 is unlikely to reset learning, while $100 to $1,000 may. Stepping a large increase up over several days works with the documented behavior.
Does adding one new ad restart learning?
Yes. Adding a new ad to an ad set is on Meta's list of always-significant edits, so it sends the whole ad set back into the learning phase.
Does pausing an ad set reset the learning phase?
Only if it is paused for 7 days or longer. In that case the ad set reenters the learning phase once you unpause it.
Where can I see my last significant edit in Ads Manager?
Add the Last significant edit column (Columns, then add it to your view) and compare it with the Results column to see how many results the ad set has received since that edit.
Does changing my cost per result goal or bid cap amount reset learning?
It depends on the size of the change. Meta lists the bid control, cost per result goal and ROAS goal amount alongside budget and spending limit as edits that may or may not be significant depending on magnitude.