A long BBC World Service article that poses this question: Is Russia arming the Afghan Taliban? Which concludes:
Moscow's reappearance in Afghan affairs is largely designed to irritate the Americans.
Link:http://www.bbc.co.uk/news/world-asia-41842285

It builds on an interview of General John Nicholson in late March 2018 and I cite it in part:
We see a narrative that's being used that grossly exaggerates the number of Isis [Islamic State group] fighters here. This narrative then is used as a justification for the Russians to legitimise the actions of the Taliban and provide some degree of support to the Taliban. We've had stories written by the Taliban that have appeared in the media about financial support provided by the enemy. We've had weapons brought to this headquarters and given to us by Afghan leaders and said, this was given by the Russians to the Taliban. We know that the Russians are involved.

(At the end) This activity really picked up in the last 18 to 24 months. Prior to that we had not seen this kind of destabilising activity by Russia here. When you look at the timing it roughly correlates to when things started to heat up in Syria. So it's interesting to note the timing of the whole thing
Link:http://www.bbc.co.uk/news/world-asia-43500299