Odin - Inspector and Serializer often causes problems with Animancer's Transitions by drawing the Inspector differently from the way Unity does it which prevents Animancer's own Inspector code from working properly. Unfortunately, Odin's documentation doesn't give any guidance on how other systems could improve their compatibility so the best way to avoid such problems is usually to just place a [DrawWithUnity] attribute on whatever field is causing the issue and contact Odin Support if that doesn't help.
[InfoBox("If you ever experience trouble with one of Odin's attributes, there is a good chance that DrawWithUnity will come in handy; it will make Odin draw the value as Unity normally would.")]
public GameObject ObjectDrawnWithOdin;
[DrawWithUnity]
public GameObject ObjectDrawnWithUnity;