Occasionally, health is completely desynced from the server (especially if you have recently healed).
For example, I just did a quick battle where I started at 0 hp, even though my planeswalker was at full, and hadn’t been used in hours. I lost as soon as I took 1 point of damage.
Another time, after healing up to 40 hp, I started a story quest only to begin it at 18 hp.
Either show a warning or resync the health totals BEFORE locking you into a fight you can’t possibly win.
I had the same issue. Did my first Quick Battle with a 46HP Planewalker, won but took some damage. Ended with 10HP, so I took a few other planewalkers through some Story missions.
Went back to the Quick event part, saw that my Planewalker healed again to 46/46 and started a new quick event. But when the match started my health was back to 10 hp ( did get my opponent down to 4HP) and thus lost the match.
I found the issue (or what I assume to be the issue).
Natural healing doesn’t seem to work if you do back to back pvp missions. Using a health pot or leveling your planeswalker does work.
I suggest restarting your game before every pvp mission in case you had waited and used natural healing.