In Gwoyeu Romatzyh (国语罗马字), the first tone uses the basic spelling except when the initial is a sonorant (响音) m, n, l, r.
ba (八) bar (跋) baa (把) bah (爸)
Sonorants mark the first tone by adding
h an after the initial (聲母) and use the basic spelling for the second tone.
mha (妈) ma (麻) maa (马) mah (骂)
Tone 1 does indeed seem to be less common for m, n, l, r. For example, the syllable nü with the first tone (nǖ) does not appear to be a word.
Is this a real pattern? Is there an explanation for it based on how Standard Mandarin evolved from Middle Chinese (中古汉语)?