Nothing to fix here, this is the correct behaviour. Because Magus of the Moon changes the subtypes of permanents, it applies in layer 4. Ability losing only gets to apply way later in layer 6. So when going through the layers of applying continuous effects, type changes are applied before the ability can be removed by another continuous effect. In short, ability losing effects cannot stop type changes

And it gets even worse. Due to rule 613.6 once an effect starts to apply, it gets to apply fully, even if the ability generating the effect is lost during the process. So if a type change effect also had a power/toughness changing part, that p/t change will also still happen, despite being a layer 7 effect.

613.6. If an effect should be applied in different layers and/or sublayers, the parts of the effect each
apply in their appropriate ones. If an effect starts to apply in one layer and/or sublayer, it will
continue to be applied to the same set of objects in each other applicable layer and/or sublayer, even
if the ability generating the effect is removed during this process