Having recently experienced a similar issue with "filler" words that mysteriously disappeared in a transliteration pipeline, I'd guess that this is due to an ar

Having recently experienced a similar issue with "filler" words that mysteriously disappeared in a transliteration pipeline, I'd guess that this is due to an artifact in the training data. Most likely the STT engine was trained on transcriptions created by lazy humans who simply wrote "4" when they heard "four o'clock", so the engine learned that "o'clock" is a speech disfluency just like "umm" that usually doesn't appear in writing.