From Tchaikovsky's harmony textbook, "Guide to the Practical Study of Harmony", page 121:
Concerning example 323 (image below) in the 9th bar bass voice (red circle):
Why is this jump from "a" to "f" allowed for strict part writing in four-division time? It's clearly not a passing tone, so by elimination it must be a changing tone. But how can this be? Changing tones he uses correctly in the blue circles in the image based off of the rules he provided. Is this just a changing note entering by a skip which is listed in the examples of page 113? It doesn't seem to match what Tchaikovsky provided in his example.
The rules for four-division time are included below the score segment image for reference (section 122).