In general, the legal effect of adoption is that the adopted person becomes the legal child of the adoptive parents and severs all legal ties with the biological parents, unless otherwise provided by law. Therefore, if your father was legally adopted, he no longer has any legal relationship with his biological father, and the land purchased in his name before adoption would not automatically become the property of his biological family.
However, the legal status of the property may depend on the specific laws and circumstances of your jurisdiction. In some cases, property purchased in the name of a minor may be considered the property of the undivided family and not the property of the individual. After the minor reaches the age of majority, the property may continue to be held as undivided family property, or it may be partitioned or divided among family members.
It is also possible that the partition suits filed by your father's biological family may have some legal effect on the property. Without more information, it is difficult to say what impact these suits may have on your father's claim to the property.
If you have documentation showing that the land was purchased in your father's name before adoption and registered in his name, this may be evidence that the property belongs to him. However, the final determination of property rights would depend on the specific laws and legal principles applicable in your jurisdiction, as well as the evidence presented in court.
Again, it is strongly recommended that you consult a local attorney who can review the specific facts and circumstances of your case and provide legal advice on your father's property rights.