I don't think there is any issue. And certainly no reason to be worried to much about this as your travel date is in March. It is very common that you can't enter e-tickets issued by third party/OTA on the web-page of the issuing airline. Most likely the AS flight or the OTA issued ticket screw things up - but it doesn't mean that there is no ticket. When you have the time and opportunity give MU a call and inquire if everything look normal/if there is a ticket - most likely the answer will be "Yes" but it may be that MU refuses to deal with this ticket as it has been issued by OTA; then you can't do anything viz-a-viz MU. But if the OTA has already confirmed to you that everything is OK (and AS webpage confirms this), I see no reason to question that and worry about it. IMO you are way overthinking this.

Ignore the comments of the previous poster about separate tickets. Clearly you are on a single through ticket as purchased by you from the OTA.