Campaign has a "Visits Web Page" trigger and a wait step in the flow. Leads were showing as being in the wait step, but are not shown in the Campaign Members tab
The leads do not show up in the Campaign Members list because they are anonymous. Anonymous leads can trigger "Visits Web Page" but do not show in the campaign membership. They can go through the flow, including going into wait steps.
To prevent this, add the filter "Email Address is not empty" to the Smart List for your campaign. This will prevent anonymous leads from being pulled into the flow.