i really like the exp share idea..
but if someone made healing potions, and share em, than same logic, yea?
like how no one else yet experienced this situation?
According to earlier in the series, it's only if your skill has a direct impact on battle (supporting with skills). Just making the potions isn't enough if we're taking the series' logic at face value. If you shared one and they drank it in a dungeon in preparation but they still needed to be healed by the healer, then your potion didn't amount to much in the grand scheme of things so you likely wouldn't get any XP. They would have to be in a situation where the healer couldn't help or where there isn't a healer, and your potion was one of the reasons they won their fight.
Then you'd have to realize that your potion is the reason you got the XP otherwise it's just gonna confuse you and you're not gonna think about it after a while.
Also healers are a thing and since healers getting XP is considered common sense, we can assume it's a common class too. It's also possible that items like potions are made by recipes found in dungeons (like the elixir earlier) and not skill related, which could be another reason why.
Another point is that the more busted classes seem to start off REALLY bad, so even if you could make potions to heal people, they'd probably suck worse than what you could buy from someone who picked it up from a dungeon. In other words, the only way to level up would be other people willing to invest their safety and health into you.
All things considered there could be a number of reasons why. It may very well be a plot hole, but we need more information about the world before we can say for sure.