Sorry for the poor source links, I only did a quick Google. Disclaimer: I am from New Zealand, and sometimes how we say things is special, and sometimes how I say things is even specialer.
In my linguistics courses (at a university in the United States), professors pronounced "marked" as a single syllable.
Generally, when indicating we want to give an E that is normally silent its own syllable, we use the grave accent. E.g., for "marked" we would write "markèd".
At least in General American dialects, "marked" is rendered as /markt/ because of rules about vowel epenthesis and consonant voicing assimilation. /markd/ would indeed sound odd to the American ear, I think. But when you explicitly add the vowel back in, the voiceless /k/ no longer forces the /d/ to assimilate its voicing characteristic, so you get /'mar.ked/. (I am using informal IPA because I'm too lazy to go copy/paste the specific symbols, but in this context they would add little anyway.)
Is marked pronounced as two syllables, mar•ked, in the context of linguistics? I am guessing it would definitely be in British English[2].
[1] https://m.youtube.com/watch?v=vrZpjr44c1w
[2] https://www.reddit.com/r/linguistics/comments/dwo5gj/is_it_a...
Sorry for the poor source links, I only did a quick Google. Disclaimer: I am from New Zealand, and sometimes how we say things is special, and sometimes how I say things is even specialer.