Hi Jessica,
We recently changed the default value of the reviewable low priority threshold from one to zero, which means that flags that were only visible using the (any) filter will appear when the Low filter is applied. We realized this default was confusing staff members, so every site can adjust it to fit their needs now.
Under the hood, we assign a numeric value to each priority filter (Low, Medium, etc.), which is re-calculated daily as the range of scores a flag can get changes over time. The reviewable low priority threshold setting value is always the value of the Low filter, and we calculate the Medium and High filters using the 50th and 85th percentile of the reviewables with a higher score.
We observed that these thresholds tend to converge near zero on sites with a large number of low-score flags, making them unusable. These changes prevent that from happening.