After setting FCV to 3.6 on 3.5.12, the mongo shell session appears as from the repro. This is different from FCV 3.4, which would give the following response:

WriteResult({
	"nMatched" : 0,
	"nUpserted" : 0,
	"nModified" : 0,
	"writeError" : {
		"code" : 16837,
		"errmsg" : "cannot use the part (a of a.b) to traverse the element ({a: 1.0})"
	}
})

In the example, field 'a' can actually be nearly any data type including a number, a string, an array, NaN, null, or undefined. This difference can also be observed regardless of whether multi or upsert is set to true / false. FCV 3.6 would return nMatched as the number of documents matching the <find> portion of the query.

The WriteResult given in FCV 3.6 may not accurately reflect a reason (or reflect a reason at all) for why no documents were upserted / modified, especially when upsert is set to true.