Entry 3734 of the 万葉集 reads as follows1:
等【ト】保【ホ】伎【キ】山【やま】 世【セ】伎【キ】毛【モ】故【コ】要【エ】伎【キ】奴【ヌ】 伊【イ】麻【マ】左【サ】良【ラ】尓【ニ】 安【ア】布【フ】倍【ベ】伎【キ】与【ヨ】之【シ】能【ノ】 奈【ナ】伎【キ】我【ガ】佐【サ】夫【ブ】之【シ】佐 【サ】
Transcribed in more modern Japanese style, this reads:
It is noted that in certain 万葉集 transcriptions, the last four characters are transcribed as 左【サ】必【ビ】之【シ】佐 【サ】 （寂【さび】しさ）. However, what interests me about this alternate transcription is that the 万葉仮名 used for ビ is devoiced; generally it is read as 必【ヒ】. I do understand that, at least for the subsequent thousand years or so, the marking of voicing was inconsistent at best, but I was under the impression that in 上代特殊仮名遣【じょうだいとくしゅかなづかい】, a distinction between voiced and voiceless consonants was in use.
So why is this read as ビ? Am I wrong that a 清濁 distinction is in use generally within the 万葉集?
Note: incidentally, every other usage of 必【ヒ】 in the 万葉集 is devoiced. Sadly, the reference above is the only usage of サビシ in the 万葉集.
Note: I originally mistakenly claimed that the 万葉仮名「夫」 was normally read as 夫【フ】. This is not true and I have updated the question to reflect this.