We find that even though it is technically permissible to shave the mustache (Tur, siman 181:12), we see that most men of piety in practice do not trim it at all, even the parts that might interfere with eating. As Rabbi Yehuda Leib Nachmanson wrote to me in the name of Rabbi Yaroslavsky shlit”a: “It is not customary in Chabad to remove the mustache, even what interferes with eating.”

Regarding this question, there is a response from the Rebbe (letter dated 5 Elul 5720), and this is his wording:
“Concerning his question about trimming the upper lip (the mustache), which interferes with eating: since I have not heard an explicit ruling, there are considerations in both directions. Therefore, one should clarify the custom among Anash (members of the Chabad community) in this matter, by consulting the rabbis of Anash who know the practical conduct handed down by the elder chassidim. The Chabad Rebbes did not trim it.

‘Compassionate’ (rachum) is the second of the Thirteen Attributes/Rectifications of the Beard (Tikkunei Dikna), and it corresponds to the root of the hairs that grow on the upper lip.”
(Ohr Einayim, p. 229)

קיצוץ שערות השפם

הנה מצינו, שאף על פי שמותר לגלח את השפם (טור סימן קפ״א הי”ב) רואים שרוב אנשי מעשה נוהגים בפועל לא לספרו כלל, גם את מה שמפריע לאכילה. וכמו שכתב לי הרב יהודה ליב נחמנסון בשם הרב ירוסלבסקי שליט״א: “לא נהוג בחב”ד להסיר השפם, גם מה שמפריע לאכילה. 

לגבי שאלה זו ישנו מענה מהרבי (מכתב ה׳ אלול תש״כ) וזה לשונו:”בנוגע לשאלתו לספר את השפה העליונה (השפם), מה שמעכב האכילה, הנה כיון שלא שמעתי הורא מפורשת, יש צדדים לכאן ולכאן, ולכן יברר המנהג אנ”ש בזה, אצל רבני אנ”ש היודעים הנהגה בפועל מהחסידים זקנים והרי מעשה רב

…

“רחום” הוא תקון ב’ מי”ג תקוני דיקנא, והוא בחינת שורש השערות הגדלים על השפה העליונה”

(אור עינים עמי רכט)

https://asktherav.com/6907-are-men-allowed-to-trim-their-mustache-when-it-interferes-with-the-food-that-goes-into-their-mouth/

The Rebbe intentionally left it open. Do what chassidim do. If the eltere chassidim you look up to do it, then fine. If not, don’t be more lax than them.

So it’s not vague – that IS the answer!

Rabbi Leibal Groners reply to my query: “The Rebbe never trimmed his mustache, neither did any of the Elder Chassidim or even young ones. there were individuals that did but a very small number.”

https://shulchanaruchharav.com/trimming-mustache/

Some say you can cut the mustache, some say (but alsoen courage) only the sides, some still avoid it… I’m more confused now then I was before the article.

I guess this raised awareness that this is a question, but one without a clear answer?

At minimum the “custom of elder Chassidim” could have been conveyed…