Update: After further clicking around on the website instead of the CX app, this was the message shown to me:

"Based on the information you provided, additional visa details for your Passport xxxxxxxx are required by local immigration officials. Please ensure all documents and corresponding visa details you entered are correct and up-to-date."

So it would appear that the CX system no longer knows that citizens of a neighboring ASEAN country like me don't need a visa to Malaysia - despite me having flown CX on this exact sector many times in the past without any issue.