I would think that the primary cause of that breakage would be the extended chain rather than X2 being in the mix if termination occurs at X1. I'm amazed at just how much struggle there has been both for hosting providers and users to support using multiple intermediate certificates. GoDaddy recently dodged the issue entirely by jumping from R3 with DST Root CA X3 assumed in trust store directly to R3 with ISRG Root X1 assumed in trust store.