what you might have is a few noisy users, or some bad experiences which have now shifted to another variant

you could try seeing if playing with winsorization / capping values

In the context of GrowthBook, winsorization is used to prevent large outliers from having an outsized effect on experiment results. This is particularly useful when you have metrics like revenue per user, where a single large purchase can skew the results. By capping the aggregated value for each user at a certain threshold, you can still count the outlier's contribution without letting it dominate the outcome of the experiment.

Yes, it is possible to manually add a new experiment phase with a start date in the past. If you don't have the ability to choose a date in the past when you create the new phase, go ahead and let it default to today, then once the new phase appears you can click the "Edit" button and modify the start date. (This is what happened to me just now when I tried it on a demo app.) Keep in mind that when you set a new start date for a phase, it is primarily for analysis purposes and does not retroactively affect the traffic that was already exposed to the experiment.