Alternating Parity
Odd, even, odd, even — nobody sits next to their own kind.
Every pair of neighboring digits must differ in parity, all the way through — like 12, 123 or 505050. Two digits minimum; a lone digit has nothing to alternate with.
Your record
Its probability structure is textbook-clean: the first digit is any of 9 nonzero choices, and every digit after that has exactly 5 options — the five digits of the opposite parity. Each extra digit cuts the odds exactly in half. So the k-digit count is always 9×5^(k−1): two-digit numbers hit it exactly half the time (45/90), six-digit ones 28,125/900,000 — precisely 1/32.
There are 35,145 across the whole range, about 3.5% — second only to Zigzag among the sequence features. Length is its only enemy: ubiquitous among two-digit numbers, roughly three-in-a-hundred among six-digit ones.
Trivia: every Fully Ascending and Fully Descending number hits Alternating Parity automatically — a step of exactly 1 always flips parity. The mirror brothers 123456 and 987654 are also the two tidiest alternating queues in the range.