What you have is called “fruit float.” When the jars of jam are very hot and there is no jell yet, the pulp, which is lighter than the juice, is able to float to the top of the jar. Strawberries are prone to fruit float although it doesn’t always happen. Other fruits can have fruit float also. You are not doing anything wrong. However, if you want to discourage fruit float from happening, you can mash your fruit a little more and you can cook your fruit for a little while before you bring it up to the full boil. You could even add a little of your sweetener to cook with the fruit, still leaving enough sweetener to adequately disburse the pectin powder without going over the sweetener limit.
In the future, when you take the jars out of the water bath, leave them for about an hour to start cooling and seal. Then come back and check to make sure they are all sealed. If you see that you have fruit float, turn the jars upside down to force the pulp to redistribute through the jar. Come back in about 45 minutes and turn the jars right side up to once again force the pulp to redistribute through the jar. Check again in another 45 minutes and if you have a distinct dividing line, turn the jars upside down again. Turn the jars right side up again in about 30 minutes. You always want the jars to end up right side up. By keeping the pulp well distributed throughout the jars, there will not be a dividing line when the jell finally starts and locks everything into place.
If your jam has jelled in a separated state, you can gently stir the pulp and juice back together when you open the jar to eat it. Separated jam in sealed jars will store safely.