This is a baseless theory, but I think the figarland family is kind of disgraced because of Shanks being associated with Roger. But idk though
Nah, it's not baseless, it's a good theory I support.
Look at how Homing got treated for leaving Mary Geoise. Since Uta wasn't a Pirate, the Gorosei had to respect the chance she was the daughter of a Celestial Dragon, and couldn't make a direct move on her.
Doffy was chased out of Mary Geoise when he tried to return.
Shanks is probably the single greatest stain on the Celestial Dragons. Garling's status was already cemented by God Valley. Pranks would have grown up hearing all about how his twin brother was the scum of the earth for following Roger.
We know the Celestials are dirtbags, and I imagine the families compete with each other in real life the same way they did in the human hunt.
If the other families found out Garling, the most powerful Holy Knight at the time, had a son being raised by Roger, a notorious pirate, they would spread the information to ruin Prank's reputation.
Shanks is 39; God Valley was 38 years ago; Roger became King of the Pirates 25 year ago. Pranks would have been Shanks age, and, since Roger was a famous pirate, Shanks would have been easily seen by the WG/Cipher Pol. Most likely, Cipher Pol attempted to acquire Shanks, but he resisted.
Because Pranks looks exactly like Shanks, he would have been compared to everyone around him, and mocked ruthlessly. It looks like Pranks decided to focus on becoming strong as possible, to overcome this.