I would find this pattern much more reassuring than concerning. A CA 19-9 change from 56 to 61 is only about a 9% increase and, by itself, is not evidence that the chemotherapy has stopped working.

Your overall trend is:

466 → 573 → 481 → 326 → 273 → 139 → 108 → 90 → 56 → 61

That's roughly an 87% reduction from 466 to 61. More importantly, your CT is showing objective improvement: the pancreatic tumor decreased from 6 × 5 cm to 4 × 3 cm, the liver metastases are no longer visible, and the lung disease is stable or decreasing. Imaging generally carries considerably more weight than one small tumor-marker fluctuation.

CA 19-9 also doesn't have to behave like a smooth mathematical curve as it approaches its lowest level. There is no requirement that it gradually slow down before reaching a nadir or before eventually rising. It can plateau, fluctuate up and down, fall again, or remain relatively stable for some time. Inflammation, biliary issues, liver conditions and changes in blood-glucose control can also affect CA 19-9.