When Aura of decay is converted to bleed with “Blood Font” the damage taken rises by a large percentage. Description of Blood Font says all poison resists should be converted to bleed resists.
Either the 30% bleed damage from all sources reduction isn’t working or the 50% self damage from aura of death is not working when converted.
Worth noting that bleed (53) as an ailment has a much higher base damage than poison (28).
That could be the source of why it feels so much more damaging. Also worth testing it in a “sterile” environment. With no sources of poison nor physical damage on your character.
However I wouldn’t doubt there might be some bugs.