https://imgur.com/lOHZ09Z

The level bar at the top, or whatever it's called shows x/75 for classic, instead of 300.

This is because C_TradeSkillUI.GetBaseProfessionInfo() lies to me. Obviously, there must be some other way to get the 300 value but I'll have to figure out where.

For one of my low level characters it returns 150 and for another it returns 225. These were the correct values in 9.2.7 but I guess you don't need to train the intermediate levels in 10.0.0.

ScanTrade: GetBaseProfessionInfo()= { ['skillLevel'] = 147, ['professionID'] = 185, ['skillModifier'] = 0, ['profession'] = 5,
['isPrimaryProfession'] = false, ['professionName'] = Cooking, ['maxSkillLevel'] = 225, ['expansionName'] = Unknown }"

Skillet-5.00-alpha4 changes the way I determine the maxSkillRank. If this doesn't fix your character, please open your profession and execute the following:
/dump C_TradeSkillUI.GetChildProfessionInfos()

My level 10 toon went to the cooking trainer and learned "Cataclysm Cooking" which has a maxSkillRank of 75 so his bar is "messed up" in a different way!