While playing Skyexchange modpack (Botania r1.9-327), I've noticed that when Rosa Arcana has taken whole EXP level, it restores only ~50% of EXP bar (at ~30lvl and up) and it restores more than a bar at ~5lvl or lower (the highlighted part of Exp bar goes waay to the right).

I've quickly looked in code here on GitHub, and looks like 'ExperienceHelper.java' uses old equations for figuring out exp amount. It should be something like (at least according to Minecraft Wiki):

if (level > 0 && level < 17)
    return (int) (level * level + 6 * level);
else if (level > 16 && level < 32)
    return (int) (2.5 * level * level - 40.5 * level + 360);
else
    return (int) (4.5 * level * level - 162.5 * level + 2220);

Cheers.