CA19-9 of a single reading is not of much value at this point. It is only a single datapoint in time and is too early to indicate a trend when chemo is just starting. It is a well known, documented fact with mode of action elucidated by researches as to the cause of the observation in its rise at the start of chemo.

This scientific paper will explain about tumor burden, CA19-9 and the rate-limiting factor of the liver enzyme that has to process a growing backlog of CA19-9 being released as chemo begins causing cell death through DNA damage resulting in the process of apoptosis.

Increase in CA19-9 in early chemotherapy treatment cycles
https://www.sciencedirect.com/science/article/pii/S2468294221000952
(This was the article I was looking for topost to your DM inquiry on Reddit).