In Plextools 2.19a, the DVD Read Speed Settings (DRSS) detects the current speed setting. So while you are doing QCheck, the DRSS changes to 2x CLV (note: it's CLV not CAV). If then you pop in another disk and read with DVD Decrypter (assumed that speedread was enabled, max speed), you should see the DRSS changes to 5-12x CAV or 6-16x CAV again. Are you getting the same?
So I think it's not a problem.
The drive reads single layer (SL) DVD-ROM at 6-16x CAV, Dual Layer DVD-ROM at 5-12x CAV, DVDÂ±R 5-12x CAV. That explains when you set DRSS to 6-16x CAV, it will fall back to 5-12x CAV because it detects a DVDÂ±R.
Edit: According to the spec, 712A can read DVD-ROM 6-16x CAV, so I guess it means both SL and DL DVD-ROMs.