If I'm not wrong, when a definite article (הא הידיעה) comes before a Kamatz Katan (as in האמנים), it also becomes a Kamatz Katan, which in case it should be pronounced Ho'ománim rather than Há'omanim. Anyway, I'm not %100 sure I'm right anyway. I have encountered similar occurrences in the bible and I think that's the rule, hope to find one one day and come back here to comment one...
Well, you confuse this it with the one-syllable prepositions used without article before a Chataf-vowel. "To an/in an/like an artist" would be in traditional style לׇאֳמָן [looman], בׇּאֳמָן [booman] and כׇּאֳמָן [kooman]. But "the artist" and "to the artist" would be הָאֳמָן [haoman] and לָּאֳמָן [laoman]. Not that you do not see the difference between [looman] and [laoman] even with niqqud!
Well, the difference is that לָּאֳמָן [laoman] has a usual קָמֵץ, but לׇאֳמָן [looman] a קָמֵץ חָטוּף, which is used in closed, unstressed syllables, but is here accompanied by an auxiliary חָטֵף after a guttural (Silbensprengung in German), like f.e. in the pseudo-dual צׇהֳרַ֫יִם noon. This is a sort of vowel assimilation, where [ləʔoman] became [loʔoman]. "Common practise" of course is to get rid of these intricacies. PS. The pronunciation of קָמֵץ with two different timbres is of course against the prescriptive Tiberian tradition and of some considered erraneous or suspect (Ibn Ezra), but is part of another (Babylonian, were קָמֵץ חָטוּף is usually [u]?) tradition and has become traditionalised for Modern Hebrew (like חָכְמָה wisdom always pronounced with [o]) too. PPS. To pronounce תֿ as [s] (coming from the original [θ] as in thing) is a tradition of the Hebrew of European Askkenazim and has been abandoned due to the minimalistic merger in Modern Hebrew with תּ. But that is another story.
The rules of spelling without nikkud are not rigid, so it's not wrong to either write אמן or אומן.
Specifically, the Hebrew Academy recently changed the rules - with the former version, it is spelled אמן and אמנים; with the new version, it is spelled אומן and אומנים.