. . . . . often deer just plopped down in the open hardwoods, even if thick cover was close by.
And this can include fully mature bucks!
I think they will plop down whenever & wherever they feel "safe" for the moment.
In many times past, there were more deer hunters, and more open hardwoods, with those more deer hunters really stirring up the deer in the hardwoods. At the same time, these hunters were
NOT "hunting" in places where they couldn't "see" far, nor places difficult to get in or thru (such as a cut-over grown into weeds & littered with treetops.
This contributed to the belief deer didn't bed in open hardwoods, and would seek thick "security" cover. What they're really seeking most is a "safe" place to rest a while, and often that is not in open hardwoods (at least during daylight).
In many cases, deer have felt most vulnerable in open hardwoods (and fields). But when undisturbed by humans, they will in fact often bed in open hardwoods, and to the surprise of many, right out in an open field!
I am often finding deer bedding in the middle of large open fields at night, only leaving when dawn approaches. This is one reason hunting fields in the mornings can be counter-productive, as there is a tendency to "flush" out the bedded deer (not just the feeding deer), and then those deer don't return until late afternoon or after dark, only to spend their night in the fields.