Version: Windows PC

I had not played the game for a while (my hero being L 18 last I played on 1.2.6).

A few days ago, I found that the latest version was 1.2.9, and upgraded from my existing version, 1.2.6.
Then I played on and off for a few days, and then it hit me: with the upgrade, ALL my existing avatars had reverted to level 1, 9 stones to go.
I definitely remember that some were levelled (you don't get to L18 without getting a few along the way), but I cannot retroactively tell precisely which ones were at which level. But all L1, 9 stones to go to level definitely is not it...

Besides this, I just hit a freak bug, un-reproduceable:
Having spent a couple of hours trying to get some levels back on my avatars using full squads of elf archers at each start (fastest run was all 5 opponent killed with 2nd archer, first turn), in 1 battle, the instant I clicked <S> on my first archer, it TRANSFORMED info a fairy... 1 instant it was an archer, the next it had been replaced.

Ugh, I'm sorry about the leveling bug
But the 1.2.6 was quite long ago vs the 1.2.9 and such things can happen. The game also was coded long time ago, before I moved to python that saves data as text (so such compatibility things couldn't happen at all).
I've been thinking about doing another game like Magic Stones using python. If I do it, you can be sure that there won't be any upgrading problem ever from a previous saved game.

As for the fairy bug, I remember another person once told me about it, but as you correctly said it was really un-reproduceable, I've tried for days but couldn't really find out WHY did that!!