Is it common to have spot bleeding from self dissolving stitches, after a combi tuck, for several months?

Spot bleeding as you describe for several months is not normal.  Absorbable sutures will occasionally work their way out prior to absorption, however this is not typically a chronic issue that lasts for months.  Sometimes a suture can become infected and continue to cause drainage until it is removed.  In my opinion, a more aggressive debridement of the drainage site to remove any foreign body suture material is warranted.  Discuss this with your surgeon.  

Spitting of dissolvable suture material can cause the problem that you are having.  Everyone has a different response to the suture material, so it is possible that you are spitting more than normal.  However, this is not a particular problem.  Every time a stitch comes through the skin, have it removed by the surgeon.  Eventually it will stop,  There are hundreds of stitches in there.