The bug
Considering the resolution of MC-162557 as "Works As Intended", and the status of MC-200929 and MC-200937 as valid issues, there is a consensus that ghasts are treated like undead mobs. However, unlike all undead mobs, ghasts can take poison damage.
Expected result
Ghasts shouldn't be affected by the Poison effect.
Relates to MC-162557, MC-200929, MC-200937 and MC-238622.