You have Workspaces and Partitions in your instance, and a static list that used to contain certain leads is now missing those leads. The leads still exist in the database and there is no "Remove from List" activity in the Activity History for the affected leads.
This can happen when the workspace that contains the static list no longer has visibility to the partition that the lead record is located in. The lead may have been moved to a different partition, or the partitions that are accessible to the workspace may have been changed.
To confirm whether this is the case, do the following
Go to Admin > Workspaces & Partitions and locate the workspace that contains the static list
Confirm which partitions are accessible to that workspace
Go to the lead record of one of the leads that was in the static list before but is not now. See which partition the lead is in currently.
If the lead's partition is not accessible to the static lists's workspace, the lead will not show up in the static list.