As you first foil, because the foil is more efficient/less drag, it takes less sail pressure to stay going. That's a reason a lot of people starting out feel their lines are better a bit forward (although 10cm sounds like a lot). As you get more used to being powered, the lines eventually move back closer to or on where you had them before.
Something that may be happening though - I went through something similar - is with the lines forward, when you are powered up and sheeting in hard with the back hand (since those lines are so far forward), is most of us oppose the backhand with the back foot. So, when you get hit with a gust, you are putting more pressure on the back of the board which does the exact opposite of what you are wanting since more pressure on the back foot helps the board rise.
Ideally, as you get more wind pressure, learn to ease a touch on the back hand and apply more down pressure on the harness/and or lean forward. Also, start to creep your harness lines back as you are more accustomed to foiling with more pressure in the sail. You can also put your back foot a bit more forward as you see the gust coming to help get some weight forward.
I agree with swoosh about moving the mast base a couple cm forward if need be but I, personally, find lowering the boom puts more pressure on the front foot. Note how Monty and Diony talk about it boom up/mast base back to free the board and get it on the fin and the opposite to get the board on the water.
Also:
howtowindsurf101.com/correct-windsurf-boom-height-positioning/