What were you expecting to happen, and what actually happened
After the 0.36 Update, I tested spells for consistency in calculations given all the changes to Spell mana cost. Generally spells follow certain incremental gains earned from levelling based on their mana costs. Several spells however seem to have inconsistencies in their calculated output. This could be a function of choice by the developer to favor or disfavor certain spells based on ancillary abilities but more likely it is the result of the recent mana cost changes in which the spell calculations were not updated to align with the new costs. The following table shows the inconsistencies (highlighted in orange):
Generally a spell under 100 mana has a power/mastery multiple of 1x (meaning for each point of power and mastery 1 point of calculated dmg/healing/etc is gained) and an incremental level gain of 1 point. Spells over 100 mana and under 150 mana generally have a power/mastery multiple of 2x and an incremental level gain of 2 points. Beyond this things get a bit spottier since there are less spells at 150+ mana cost; but Frostbolt and Insect Plague seem to be genuine errors; the spell has a mana cost of 160, a Power/Mastery Multiple of 3x but an incremental level gain of 1 point (compared to Hand of Evil and Poison Spirit, both 150+ mana cost spells with incremental spell level values of 2 points). Something like Wrathful Smith, however, may have simply been a developer choice. (I believe Night Blade is actually a 1.6 multiple instead of 1.5).
I am providing this not to suggest that every item listed is a bug or error but rather that these spells represent inconsistent treatment and may be in error.
How often does this happen? When did it begin happening?
Data gathered since the 0.36 update
Steps to make it happen again
See chart