I'm Not a native English speaker, but I'd venture to say a few words. As far as I know, it's not a matter of which (of /lʊˈɡuːbrɪəs/, /luːˈɡuː.bri.əs/ and /ləˈɡuːbɹi.əs/) is more right or more common, but a matter of how the native English people speak.
In English, as we all know it well, a vowel does not always pronounce the same, even in an identical root or stem. The position of the stresses (especially the primary one) in a word (...) and the speed of speech are two key factors that determine the particular sound value of a vowel in a syllable or a word. Examples: ......
With regard to 'lugubrious', when you say it at a fairly low speed, it's normal you articulate it as /luːˈɡuː.bri.əs/; if you speak faster, /lʊˈɡuːbrɪəs/ is more possible; and if more faster, /ləˈɡuːbɹi.əs/, or even /ləˈɡʊbɹi.əs/ is natural.
In addition, the American pronunciation of English is closer than that in Britain to the older pronunciation before they began to diverge from each other. Example: literature.