…reasons for this?
The court recognizes the will as invalid or void only if there are good reasons. They can be:
violation of the will form;
signing a will due to psychological or physical violence;
inability to control oneself and manage one's actions due to illness;
writing a will under the influence of deception.
Each of the reasons has its own features of proof, which must be taken into account when preparing a lawsuit.
You have 3 years to file an appeal from the moment you became aware of the violation of rights. But even if the term is missed, it can be renewed. The reasons must be valid: long-term illness, business trip, service in the Armed Forces of Ukraine.