The purpose of this clinical trial is to compare drug combinations to learn which drugs work best to prevent graft-versus-host-disease (GVHD) in people who have received a stem cell transplant. The source of stem cells is from someone who is not related and has a different blood cell type than the study participant. The researchers will compare the new drug combination to a standard drug combination. They will also learn about the safety of each drug combination.

Participants will:

* Receive the standard or new drug combination after transplant
* Visit the doctor's office for check-ups and tests after transplant that are routine for most transplant patients
* Take surveys about physical and emotional well-being
* Give blood and stool samples.

This clinical trial is evaluating whether addition of navtemadlin to ruxolitinib treatment will provide more clinical benefit than ruxolitinib alone for patients with Myelofibrosis who have a suboptimal response to ruxolitinib treatment alone.

Subjects will start by receiving ruxolitinib alone in the run-in period. Those who demostrate a suboptimal response from ruxolitinib alone will then be randomized 2:1 to receive navtemadlin or navtemadlin placebo as add-on treatment to their ongoing ruxolitinib. Randomized means that subjects will be assigned to a group by chance, like a flip of a coin. The study is blinded, meaning the subjects, doctors, central endpoint assessors and sponsor will not know which add on treatment (navtemadlin or navtemadlin placebo) the subject is receiving.

ABNL-MARRO (A Basket study of Novel therapy for untreated MDS/MPN and Relapsed/Refractory Overlap Syndromes) is an international European-American cooperation providing the framework for collaborative studies to advance treatment of myelodysplastic/myeloproliferative neoplasms (MDS/MPN) and explore clinical-pathologic markers of disease severity, prognosis and treatment response.

ABNL MARRO 001 (AM-001) is an Open label, phase 1/2 study within the framework of the ABNL-MARRO that will test novel treatment combinations in MDS/MPN. Each Arm of AM-001 will test an active myeloid target compound in combination with ASTX727, an oral drug combining fixed doses of the DNA methyltransferase inhibitor (DNMTi) decitabine and the cytidine deaminase inhibitor E7727, also known as cedazuridine in a single tablet.