Barring questions about properly leaned mixture, and configuration, which have already been covered here, an aircraft’s climb performance (and thus service and absolute ceiling) is highly predicated on aircraft weight, CG, and density altitude (pressure altitude corrected for nonstandard temperature).
At 17,000, the standard temperature is around -19°C. Currently the winds and temps aloft forecasts at altitude in that area are ISA+5. So that means a temp of about -14°C at 17000’, which gives a new density altitude of 17500’.
I have no idea your weight and balance situation, but the heavier a plane is, the lower it will top out. The more forward your center of gravity, the lower it will top out. Keep in mind as well that your best rate of climb speed (Vy) decreases with altitude and best angle of climb speed (Vx) increases. Where they meet is your absolute ceiling - you will no longer be able to climb and any further increase in AoA will induce a stall. And remember, this is variable based on the conditions I stated.
So you’re already playing on the margins of the plane’s ceiling, but now you’ve added ~500’ to the altitude it which “thinks” it is, plus unknown weight and balance factors. I’d say the fact it almost got to 16,000’ with a notch of flaps in (don’t do that) is pretty good.
For anybody else testing this, it helps to know the density altitude (altimeter setting and outside air temperature should suffice) and your aircraft weight and balance.