This trial is testing if a combination of two drugs, ASTX727 and venetoclax, is more effective than ASTX727 alone in treating certain bone marrow cancers. It focuses on patients with CMML and MDS/MPN who have too many immature blood cells. ASTX727 helps produce normal blood cells and kills abnormal ones, while venetoclax blocks a protein that cancer cells need to survive. Venetoclax is an anticancer drug used to treat lymphomas and leukemias, but it has severe side effects.