Noticed this while working through the fraud filters. On "reject" in the Phase_Validate case, the page is redirected without the payment parameters which causes the error page attached below (url: https://localhost:9001/index.php?title=Special:AdyenCheckoutGateway&appeal=JimmyQuote&ffname=error-cc&gateway=adyen). On digging deeper, it was realised that the session was lost at some point in the process.
WIP Gerrit patch: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/DonationInterface/+/736651/
To reproduce locally
Make a donation attempt that triggers the filters that gives a Referrer risk score of 70