Hey, Fretful. It's not a bug, you are pretty much correct in your math - the upgrade is a total of 20% from the original 20% Heal, which amounts to 24% in total. I've passed feedback on this to the Dev Team, we'll consider if there is a better way to relay the situation in the Skill descriptions.

Marius, this has been reported at least as early as January 1st in a post in bug report about Hexweaver healing skill. It is a very clear case of wrong description and it should have been urgently fixed way before official release. This is a huge issue as you're in a phase where a lot of new players are coming to the game and they all understand the same erroneous thing,  only to realize after a while that they didn't get what they were promised, especially if after spending money and time to acquire skill tomes that they would probably not have chased if it was for a ridiculous 4% upgrade. 

Either update the skill upgrade descriptions so that they become honest, or change the way they're calculated, but this is URGENT. You could be facing numerous refund requests, or even well founded complaints to Google and Apple and it would be a shame if it should impact negatively a promising game like this one. Honestly I can't understand that this problem hasn't been taken more seriously in the past few months at least... 

Please do report the situation to the devs as they've put the game in a very bad situation with this. 

There is another 'bug' with the Warpriest's wording.  Once ascended, the text says that In addition to the 20% heal, all Ally's except the target are healed by 10% of the target's HP.  Instead each non-target ally is healed by 10% of that non-target ally's HP.  The faulty text implies you can be more strategic with the heals than you really can.  

In the unascended skill description it also isn't mentioned that the cooldown will increase from 3 turns to 5 when ascended.