Ertz recanted his original attribution to Jan the Elder and in the Jan the Younger book attributes it to him. Peter Sutton, in the Age of Rubens show, gave it back to Jan the Elder. Jaffe, in Rubens Opera Completa, also gave it to Jan the Elder, and dated it c.1617.
There was definitely an autograph collaboration between Jan the Elder and Rubens of this subject, because it's listed in his estate inventory in 1626. This work has a good possibility of being that picture. If so, it could be same work sold by JBII in 1627 to Paris, but that might also have been a JBI/Van Balen version of same subject which is now in London. We do know, also from Jan the Younger's account books, that he and Rubens did collaborate on this subject as well. It is equally possible that the work was left unfinished by Jan the Elder on his death (with figures already done, see below) and completed by Jan the Younger.
Evidently in this and the other version of it, it is clear that the figures were painted first; thus, panels were primed in Rubens' workshop and figures painted there, before being sent to the Brueghel shop for landscape.
This panel bears panel-makers mark of Michiel Vriendt, who supplied many panels to Rubens.
Sotheby's sale catalogue says that "the brushwork lacks the shimmering, nervous line of the elder Brueghel" and from their reproduction this does seem true.
Variant--with landscape oddly reversed but figures in same position--is in Schwerin, Staatliches Museum (see Ertz, p.502). Attributed to Jan the Younger and Rubens' workshop. That work oddly reverses the "narrative" so that in present painting Syrinx is running out of the swampy forest into the clearing whereas in Schwerin painting she's running out of open space and into protective forest/swamp. There is also a copy in Milan, Brera (inv. #623, panel, 43 x 33).