Whe the system identifies you as a smurf (presumably due to a big winrate at early levels), it can overshoot your 'true' mmr somewhat as it tries to find an upper bound for your skills.
If you're doing fine and winning around 50% of the time with gold/plats, then perhaps you are at that level of gameplay (which should be interpreted as 'the level of gameplay that some golds/plats play at in normals' and not evidence that your main deserves a higher rank), but until you've had dozens of games with around the same level of players for things to settle I wouldn't read into it too much.