Looks like something is distrupting session attribution.
Traffic sources and campaigns are tracked as you expect? That is: platform clicks are more or less aligned with visit data?
If this is the case, then the most likely cause is there’s something that writes a new visitor ID for the user coming back from the payment gateway.
I STRONGLY recommend doing a full simulation of a user coming from a campaign and ending with a conversion, while using this awesome tool by David Vallejo
It’s like GA-GTM debugger… but on steroids and it supports Matomo!
Should be useful to make the right observations.
If you also would post updates about this, that would be great.