In canvas painting, there's a pretty hard rule: Never paint acrylic over oil. In mini painting, you are using much less paint with much less solvent, so it's a bit more flexible. But nonetheless the rule exists for a reason...
Oil dries very slowly (obviously - I assume you've noticed this already

) and can interfere with an acrylic layer on top, making it do weird things.
Acrylic literally dries - the solvent evaporates, leaving the pigment and binders which attach it to the surface. Oil does not work this way. The oil does evaporate, but the paint also undergoes curing - ie a chemical reaction. The paint will oxidise with contact with the air, further hardening, even when evaporation is finished.
All that is to say - oil paints 'dry' quite a while after they feel/ appear to be 'dry'. If you apply acrylic over the top of oil that isn't fully cured, it can make weird things happen. I assume that is what happened to you ! a small patch of oil paint that hasn't fully cured that is doing weird things to the subsequent layers. When buried under more and more layers, it will never properly dry.
I think the only way to fix it would be some solvent and cotton bud, and carefully remove all the paint on that area... That or leave it for a few weeks and try another layer; you never know...