Without a video clip of your performance, it will be hard to diagnose a problem. However, I can give you some tips that might help you, as this kind of problem is fairly common - usually, it is beginners that have these problems.
Sometimes, the heel lifts before you take off for a step; other times, the heel lifts as you strike even though you landed properly. You didn't indicate the timing at which you lift the heel, that is an important set of troubleshooting steps. Personally, I don't always mind the heel coming up in order to take a step, but some instructors have differing opinions.
If you are rushing to strike before you get into your stance, that can pull you forward - that can lift the heel. The solution is to slow down, and time your strike to land with the step, if that is what is called for in your form.
At the moment of your strike, your body should be settling down - as in "downward". So, your strike goes forward while the body sinks down.
Lower your shoulders. Rising shoulders encourages raising the body, which encourages lifting the heel.
Lower your chin. An abnormally uplifted chin can encourage rushing the strike, which can lift the shoulders, which can raise the body.
It could be you're doing all else correctly, which means, you're just lifting for the strike or for the take off on a step. This means a bad habit has settled in. The fix for this is to practice at an exaggerated slow motion as you go through your forms, and focus on keeping the heel down. To undo habits is difficult; you'll need to do this more than you think. How much time is hard to say, it depends on how comfortable you felt while lifting the heel; you'll need to overcome that comfortable feeling and replace it with not lifting the heel, and so now you have to practice enough for it to be comfortable, then practice enough for it to become habit.
As to this (the bad habit) this is often seen in young children who have walked on the balls of their toes. This is a bad habit for them as it leads to poor posture, and doctors are keen to have the child settle onto the heels. You'll hear parents remind the child "off the toes" or "don't walk on your toes". As you get older, these habits tend to go away, but depending on how long it has been occurring (if this describes you), and how old you are now; but, this current habit could have been borne from this early childhood habit. You see it in people who, as they walk naturally, they step and instantly lift the heel. I'm not sure if this is a bad habit, as far as walking and posture is concerned; but I notice that people who do this kind of bounce as they walk are often the same ones who lift heels as they step.
Now, I realize you qualified your problem with kata, but, you didn't say you weren't having problems when not doing katas, and, it's possible you are. So if you don't have problems outside of kata, then this doesn't apply of course:
But when you strike - usually in problems like this, it will be a reverse strike (the hand opposite the lead foot) which when over-reaching, it pulls the rear foot up. In this case, you need not think you have to reach to hit your target. If you do, you are off-balance, and then could be used against you. If you are lifting while "air striking" (that is, striking without a target) then you need to think about your target to be imagined much closer than you are. If you are lifting the heel while striking a target (hand-held or a hanging or standing bag) then be closer to the target so that you don't have to feel you are reaching.
