Reported here: matomo-org/matomo#19318 and also experienced this myself, if the MarketingCAmpaignsReporting plugin was not active from the beginning (but activated later, while data was already in the system).
The report Acquisition -> Campaigns reports only numbers since the activation of the MarketingCampaignsreporting plugin for overlapping timeframes. For timeframes before the activation, report data is still correct.
The report Acquisition -> All Channels -> Campaigns still reports correct numbers, also for overlapping / intersecting timeframes.