Minecraft: 1.12.2
Forge: 14.23.5.2847

Mods: 2
jei_1.12.2-4.15.0.291
levelup2-1.2.2

Level Dupe
When you add levels to your skills but don't click 'done' and add levels with your XP... you will get a refund of the suppose-to-be-spent levels.

Example
I have 5 Stored Levels, I put them into Stealth and raise it to 3/10 which will cost me 5 Levels, leaving me with 0 Levels. Now before clicking on 'done' I click on 'Add Level' to add in one level. Now... I should have 1 Stored Level... but instead it shows me having 6 Stored Levels. The Stealth Skill still shows 3/10 and the next level up for that will cost 3 Stored Levels so I'm able to level this skill up to 4/10 with 3 Stored Levels remaining.

If one has a high number of Stored Levels they will be able to level up ALL of their skills at once to their max value, as long as they have enough XP to gain Stored Levels to get back the Stored Levels that were about to be spent.

Level Lost
If you add Stored Levels into a skill and you exit the interface without clicking on 'done'. You will lose the Stored Levels that you were about to use and the Skills that you were leveling up won't increase.

The Dupe still occurs in "levelup2-1.2.3" but it's different this time. All skills cost 0 Skill Points when increasing them as long as you have enough Vanilla Levels. Each time you increase a Skill's Rank it will NOT decrease SP but you gain 1 SP per 5 Levels automatically. If however you don't have enough Vanilla Levels it will then take the SP as intended.

Example
I have 6 Skill Points, I increase Stealth by 1 which will cost me 1 SP. I do that and... my remaining SP is 7? Stealth was increased by 1 Rank so that means that as long as I have enough Vanilla Levels... I can pretty much level up my Skills almost free of charge as long as I have 5+ Vanilla Levels to spare.

EDIT
If you check very closely I can see that SP does actually take away the levels but because 1 SP is gained per increase... it cancels out the SP cost. I can see the number shift from... say 12 to 5 then to 13 when spending 7 SP.

I know exactly what happened, I'm dumb and forgot that the packet I was sending assumed you were just adding a skill point period.

Confirming that the bug is fixed as of 'levelup2-1.2.4'. Thank you.