I ran into this on my (now 50) Sentinel. I went full completionist on every planet, so I was level 20 by the end of coruscant, 26 by the end of taris, 31 after nar shaddaa, 35 after tatooine, 38 after alderaan, etc etc
started Voss at level 48 (skipped the Hoth Bonus Series because Hoth is absolutely TERRIBLE), I hit 50 before I finished the main story or my class quest line on the planet. I was saddened that doing quests at 50 doesnt give you any extra credits (unlike WoW), making Corellia completely pointless (aside from the class quest).
now on my trooper alt, I'm just doing class quest and main story arc for the planets until I'm back within the recommended level range. Once I get past the top end, its time to move on.