Peering at pictures of earlier Progress flights, these features were visible on MS-01 (62P) but not on M-28M (60P) which just has a couple of metal "pegs" instead. So it looks like an MS-series change.
Spaceflight 101 outlines the changes that were made in the series but does not actually nail down what bit is what:
Progress MS introduces a new KURS navigation system, a new radio, the use of GPS/Glonass for navigation, and the use of a proximity communications link for relative navigation. These changes will not significantly change the external appearance of the Progress except for the number of deployable antennas present on the spacecraft...
However, what looks like exactly the same equipment is seen on Soyuz MS-01 but not on TMA-20M. So it looks like it was a new feature across both the Soyuz and Progress MS series. The WP article for the Soyuz MS series includes:
The old 38G6 antennas are replaced with four omnidirectional antennas (two on the solar panels tips and two in the PAO) plus one S-band phased array, also in the PAO.
This sounds exactly like our devices. According to this detailed summary on RussianSpaceWeb, the 38G6 equipment was part of the flight control system, and is being replaced by:
...the ASN-K satellite navigation system, which could simultaneously use the GPS and GLONASS navigation satellites. The system would allow autonomous trajectory measurements by the ship's computers without reliance on ground stations.
So sounds like your "omnidirectional GPS antennae" was exactly it.
(Oddly it says the first flight of it was on Progress M-25M, not MS-01 - but possibly that was mounted differently for testing purposes? It doesn't seem to be visible on the panel tips in any photographs, which just have the old-style "peg" antennae)