in fights I remember (not 100% sure) there was no curse or sickness. moreover difference was exactly 60 (potion add +6).
when you use potion, health and spirit are stil going to max - so its not problem of lack of health before the battle.
Maybe my reply was unclear, but it seem to be exactly the reply you need. Let's do it step by step.
1) you use a stat potion, it give you increased stats for 5 fights;
2) use heal and spirit potion to get max HP and spirit;
3) at the end of the 5th fight your stat go down;
4) your HP and spirit lower to match the new value;
5) you use another potion, your stat increase but your HP and Spirit stay at the value you had before the buffing (as it always happen);
6) so you use again heal and spirit potions to get max HP ans spirit.
Instead if you follow this procedure:
1) drink stat potion. You get the bonus for 5 fights;
2) raise HP and spirit to max;
3) fight a round of combat. The bonus now last for another 4 fights and the extra HP and spirit will do the same;
4) drink another stat potion (of the same kind). Now your extra HP and spirit will last for 9 rounds, allowing you to go till the ebd of most (if not all) the multi stage dungeons.
If instead you are saying that while still under the effect of a stat potion your health is lowered at the end of every fight, it seem you are suffering from a bug.