Lead went through CDV flow step but it's not logged in the activity log
Issue
A lead shows as being a member of a campaign that had a single flow step of 'Change Data Value'. However, the lead doesn't show in the Results tab of the campaign, nor is the CDV activity logged in the activity log for the lead.
Solution
This can happen if the field to be updated was blank/null, and the new value in the CDV flow step is derived from another field, using a token, and this other field is also blank. Because the value 'changes' from blank to blank, we don't log anything in the activity log, but the lead is a member of the campaign.