Does anyone here have access to imagery that might verify this or is it probably as fictional as it seems to me?
Sure looks like uninterrupted forest to me on the imagery I normally use in JOSM.
The major change is along a railroad In the US but this grass area looks to be in the Amazon. Seems like a mistake to me though I haven’t a clue how one could accidentally do that.
The mapper seems to normally work on railroads and from the few changesets I looked at seem reasonable (I am ignorant of railway details so I could be wrong).
I guess protocol would be to add a change set comment and ask them about the grass area. But I would be tempted to simply delete the grass area if it were an area I was familiar in.
Doesn’t look normal to me, probably some iD mistake (seen already a couple of these). I guess the first step is to contact him/her (as you already did). If not answered in a week, could be safely deleted that part.