Like you, champagne was 'not available' on a longer Asian flight on MH before Christmas, where it was listed on the wine list in business class.
We had no issue with wine and gin on the KUL-BKK flight, again flown before Christmas.
If this is a new policy, I have not been able to find anything out about it on the MH website. It seems strange to lose product consistency, especially in business class, where customers are connecting.