I don't see any particular reason why it couldn't start with a vowel, anyway - Sounsyy's bountiful examples notwithstanding. Mostly because of how the names are pronounced from my recollection. Even in the case of U'odh, you wouldn't pronounce it AS U'odh - the apostrophe is supposed to be inflected with a sort of hissing sound, I think? So it's more of an /u-hh-o/ rather than an /uo/. A sort of feline glottal stop (a term I only know because of learning a bit of Arabic, I will admit).
And even then - though I forget if it's just an RP thing or an actual thing in the lore - since the letter before the apostrophe is just representative of tribe, it isn't usually used anyway. U'odh would be referred to as Odh in common conversation, if I'm recalling all this correctly. So that makes whether or not their "actual" name (as composed to the "composite" name, which includes the tribe indicator) starts with a vowel even less of a "problem."
So, I think it's just more of a matter of the Random Name Generator's pool being found lacking than any hidden lore-rule about Miqo'te naming conventions.
And even then - though I forget if it's just an RP thing or an actual thing in the lore - since the letter before the apostrophe is just representative of tribe, it isn't usually used anyway. U'odh would be referred to as Odh in common conversation, if I'm recalling all this correctly. So that makes whether or not their "actual" name (as composed to the "composite" name, which includes the tribe indicator) starts with a vowel even less of a "problem."
So, I think it's just more of a matter of the Random Name Generator's pool being found lacking than any hidden lore-rule about Miqo'te naming conventions.
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