I meant when you lose a level, you lose what you rolled on your hit die but if you had con bonus added at that level, your lose that too.
If I track ho gained per level (using Maxhp for ease)
Like L2 mage with 11 con with 8hp loses level and ends up with 4hp
Lvl2 fighter with 17 con and 26hp loses level and has 13hp left.
Using the FG thing would only knock the rolled (assume 10 on fighter) but also -1 con so he loses 10 instead of 10+3 BUT he dropped one con and had a bonus so he loses one point PER level as well. So in this case 12 points. But you way is meaner (good since levels not lost) for those with good go bonus cos if level 10 and get drained, loses (again pretend max) 10 hp ... and if first drain this fighter would lose 10+10 hp with new con of 16.
Like said I like the reduction in issue with levels and xp loss but has general bad other side effects that are like of drain “restorable” is can do so.