Online Maps takes time to redraw the map with the new marker texture.

There is one way around this:
Online Maps / Troubleshooting / Render In Thread - OFF.
Replace «map.Redraw» to «map.RedrawImmediately».

    private void OnMarkerClick(OnlineMapsMarkerBase marker)
    {
        float distanceInMeters = OnlineMapsUtils.DistanceBetweenPoints(marker.position, playerLocation).magnitude*1000f;
        if (distanceInMeters < 64f)
        {
            //change texture;
            OnlineMapsMarker m = (OnlineMapsMarker)marker;
            m.texture = markerTexureSolved;
            m.Init();
            map.Redraw();

        }
    }