Is addressing one's father or grandfather by a title such as "Al-Hajj" considered a form of disobedience (Uquq), and does this entail a sin that necessitates seeking forgiveness from the father?

Allah has made it obligatory to honor and respect parents and has forbidden disobedience to them, in accordance with His Almighty saying: (Worship Allah and associate nothing with Him, and to parents do good) and His saying: (And your Lord has decreed that you not worship except Him, and to parents, good treatment. Whether one or both of them reach old age [while] with you, say not to them [uff] or repel them but speak to them a noble word. And lower to them the wing of humility out of mercy and say, "My Lord, have mercy upon them as they brought me up [when I was] small.")

Therefore, if addressing parents by a title that does not imply reverence and hurts their feelings, it must be avoided, and one must repent to Allah and seek their forgiveness. There is no sin upon you for what passed due to your ignorance. However, if it does not contain anything that indicates a lack of reverence, then there is no harm in it.