This has happened to me a number of times in the past few weeks, most recently during RotGP today.
My planeswalker is on full life (I may or may not have needed to use a potion to heal them), I enter a level, and my ingame life total instead starts at a bare minimum (today, I started with 12 life with Karn).
This is a recent problem, I do not recall getting it before now, maybe when 2.9 was introduced?
I think it is related to a loss of connection as the level is starting. I’ve certainly had some odd effects happen when I know for certain that my connection has dropped when starting a game.
I had a similar thing happen to me. I went to start a match in TDW top left node (W/R/G) with Koth. My choose planeswalker screen looked normal, I had a green start bar and my planeswalker showed full health. I hit start and the game warned me about starting with low hp. I couldnt heal on the screen so I went to the change deck screen and it showed me with low health. I used a healing potion and started the match. When I get in I had 11 hp and promptly lost the match. I do not remember if I had used Koth in another event before this fight to cause it to have lower hp than normal.
I was experiencing some connection issues right before that fight, one of my encounters (TDW bottom left B/R/G) wouldnt end after I won, it got stuck on the rewards screen saying ending encounter and I kept getting an error saying it couldnt connect to the server. I backed out of that after disconnecting and reconnecting my wifi. I might have restarted the app after that, I cant remember.
Thanks for the info everyone. I’ll pass this along for investigation. If you all keep experiencing this, could you please also submit tickets to Customer Support regarding this issue?