Synopsis:
A) Please fix duplexing such that page one of a document always begins on the front of a sheet of paper. This means a the back page of an odd-numbered document should be blank.

B) Please let the Collate button both be visible regardless of the number of copies and let it have a default and/or remembered setting. The combination of it being initially invisible and the defaulting to the less-desirable setting results in wasted printed material.

Detail:
This is a consistent problem that sprung into existence about a year ago, near as I can tell on both Acrobat DC and Reader for Mac.

OS: It occurs on every install I have (4 different Macs, various versions of OSX but reasonably updated over that timeframe).

Printers: this occurs on every printer in the organization. All are Brother laser printers. At least 5 different models.

Duplex printing generates output that is unexpected, and unworkably so, in these situations.

Odd number of pages. Say you print a 3-page document, Copies = 2, Duplex on, Collate on.

Sheet: front page/back page. Expected output from the printer:

Sheet one: 1/2
Sheet two: 3/blank
Sheet three: 1/2
Sheet four: 3/blank

Actual output is:

Sheet one: 1/2
Sheet two: 3/1
Sheet three: 2/3

Why anyone would actually want this output is beyond me. That said, even if it was somehow desirable, why would it be the default. The only workaround I have found is to either add a blank page to the document to make it an even number of pages, or to print 1 copy many times. This is woefully inefficient.

Collation also causes this problem. Specifically, the Collate checkbox does not appear until the number of copies is increased beyond 1. Then the Collate box appear, but always unchecked. If you do not notice this, then your duplex output will be:

Sheet one: 1/1
Sheet two: 2/2

Also a wasteful default behavior, both in terms of paper and ink, but also in terms of time.

While there might be some sensible reason for both of these behaviors, I cannot fathom it. What I do know is that they product unexpected default results that are different than how other programs print and different than how Acrobat behaved until about 1 year ago.