Not a bug at all, you found the "banquet" item which refills all of your life. At the beginning of the stretch with the spiders you'll find one of the small food items that refills a health bar if one is damaged (and if you have one, two or three full bars it'll give you a coin bonus). Towards the end, a spider will come down that has the banquet if you kill it. It's easy to miss, as it only steps out when you're almost past the area but if you hold your shikigami up there long enough you're bound to hit it.
Here's a visual:
It is rather generous for a Cave game, you're right about that. Later in the level when you get to the carriage convoy, each one of those has a health item as well. They're a lot harder to get however. Also when you get to the boss, the first form has a health item that will refill a full bars worth. Destroy the...thing on the top right side of the screen to get it. Can't recall off the top of my head what it is, but it looks like a lamp or something. It's a big purple flame. Destroy the one on the top left to get a bomb.