Hello and welcome to frog forum!
I'm so sorry to hear about what happened to your frogs!
so tragic...
From what you explained, it doesn't sound like you did anything wrong.
I'd say it isn't likely that the frog could have drowned in its water bowl. In 1/4 inch of water or even 2 or 3 inches of water, the frog should have been strong enough to float and get out.
It sounds like the effects of the heat may have simply been delayed in the second frog. That or there may have been prior issues in one or both frogs.
Again i really don't think you made any mistakes here. Any frog that is delivered healthy to your home should be more than able to survive in that much water
i would definitely contact the seller and inform them of the situation because assuming it wasn't the heat then the problem is definitely on their end.
When you're looking for a new milk frog(s), Josh's Frogs consistently delivers well established quality animals. They don't have any in stock right now but they usually have more babies coming of age in early fall, if you're willing to wait. It will also give an opportunity for you to establish your self sufficient enclosure (plants can grow, springtails and isopods can get established, etc)