Perhaps part of the Nivelir-ZU (14K167) project. Rendezvous target for the secretive Sputnik Inspector satellite launched at the same time.
See Kosmos 2519. It is believed that the satellite's real name is Napryazhenie No. 1, and was probably built by the Lavochkin company. It may have carried a geodetic payload and a space surveillance (space debris monitoring) payload. However it was primarily observed in a series of anti-satellite tests with the two other payloads it was launched with:
Cosmos 2519 was launched aboard a Soyuz-2-1v rocket from Plesetsk on 2017 Jun 23 and placed in a 654 x 669 km x 98.1 deg sun-synchronous orbit with 09:54 local time descending node. The Volga upper stage was deorbited the following day. A Soyuz Blok-I stage was left in a 284 x 650 km transfer orbit.
On Jul 27 at 1200 UTC, Aug 1 at 1215 UTC and Aug 3 at about 0800 UTC Cosmos 2519 performed small (0.5 m/s each) orbit changes to lower its orbit to 649 x 669 km.
On Aug 23 at about 0640 UTC a subsatellite, Cosmos 2521, separated from Cosmos 2519 at a relative speed of about 0.5 m/s. The subsatellite was described by Russia at that time as a `satellite-inspector'. Cosmos 2521 and 2519 carried out a series of exercises involving orbital changes and mutual flybys.
Exercise 1: Distant flyby
Cosmos 2521 drifted away from its parent over the next few days to a maximum range of about 300 km and then made orbit adjustments to reverse the drift (on Aug 27 and Sep 4). By Oct 11, it had reapproached Cosmos 2519 within about 10 km. Another manuever caused it to retreat to about 50 km.
Exercise 2: Close flyby and distant stationkeeping
Further rendezvous burns by Cosmos 2521 returned it to the 10 km point by Oct 15, with approach within 2 km of Cosmos 2519 by Oct 18. It remained within 15 km of Cosmos 2519 until Oct 31, with both in a 650 x 667 km orbit.
Exercise 3: Deploy subsatellite
On Oct 30 at 0352 UTC a further subsatellite, Cosmos 2523, departed Cosmos 2521 with a relative velocity of 27 m/s into a lower-perigee 554 x 664 km orbit. As of Aug 2018 Cosmos 2523 has made no orbit maneuvers since its initial deployment. The three satellites (2519, 2521 and 2523) were registered with the UN by Russia in orbits of 651 x 683, 656 x 688, and 656 x 687 km respectively, making it hard to be sure which name refers to the lower-perigee object.
Following the deployment of Cosmos 2523, Cosmos 2519 and Cosmos 2521 began to drift apart.
Exercise 4: Close flyby by Cosmos 2519
On Dec 14 at 0900 UTC Cosmos 2519, at a range of 1000 km from Cosmos 2521, manuevered to begin an approach. At 1340 UTC on Dec 15 Cosmos 2519 flew past Cosmos 2521 at a range of less than 7 km and a relative speed of 35 km/hr. By Dec 19 the satellites were several thousand km apart again.
Exercise 5: Close flyby by Cosmos 2519
Without further orbital manuevers 2519 lapped 2521 again on 2018 Feb 3 at 0700, passing around 10 km away at about 35 km/hr again.
Exercise 6: Slow flyby
On 2018 Feb 14 at 0407 UTC it was Cosmos 2521's turn to maneuver, with a 9 m/s burn lowering its orbit to 618 x 664 km to begin a rendezvous with Cosmos 2519. The two satellites passed each other slowly at a range of about 30 km on Feb 20.
Exercise 7: Slow flyby
By Feb 27 range was 380 km; reapproach burns led to a slow flyby at a range of less than 1 km from around 0730 to 1230 UTC Mar 1.
Exercise 8: Slow flyby
Cosmos 2521 then retreated to 80 km range on Mar 6, and resumed approach to carry out another 1 km-class flyby on Mar 7 around 0700 UTC.
Exercise 9: Slow flyby
Again, Cosmos 2521 retreated to 550 km range on Mar 16, and resumed approach to carry out another slow flyby on Mar 21 around 2345 UTC, then drifted further to 24 km range by Mar 26.
Exercise 10: Rendezvous
On Mar 26, Cosmos 2521 returned to Cosmos 2519 and began a new phase of stationkeeping within 1 km, remaining near the parent satellite until Apr 30.
Exercise 11: Cosmos 2521 move to low orbit
Following the rendezvous, on Apr 30 it lowered its orbit in two large burns from 664 x 660 km to 350 x 369 km. During May and June the satellites remained in their now-different orbits without further activity.
Exercise 12: Cosmos 2519 move to elliptical orbit
Then,from Jun 27 to Jul 19, Cosmos 2519 made a series of smaller burns to change its orbit from 644 x 659 km to 312 x 606 km and then up to 317 x 664 km.
Exercise 13: Cosmos 2521 second orbit lowering
The day after the final Cosmos 2519 burn, Cosmos 2521 lowered its orbit even further, to 292 x 348 km. The timing of this change is clearly not coincidental, but as far as I can tell the two vehicles did not make any close approaches during this period. There has been no further orbit change activity since Jul 20.