Thanks for the tests 
I did try the C208b and like you found no issue.
But the test on C408 did not go as well.
- starting from the runway
- flc to 10.000ft at 105kt
- the plane climbs (a bit faster, at 110kt) to 10.000ft
And this is where the issue lies : after stabilizing at 10.000ft, the C408 will accelerate but not maintain the 10.000ft, and it climbs up. It’s like it overshoot the tolerance of leveling (something like 200ft) and then decide “it will keep the 10.000ft the next time it crosses it”. But at the 13.000ft mark, the plane stabilize at if “going too far from 10.000ft is too much”.
I think this is the same bug that has been reported here by @PreachingPigeon. The issue is that on this bug report, the OP stated that for them, they had no issue “Free Flight”, which we do.
I upvoted both bug reports and I’ll comment back in the mentionned bug so that they are aware that we have an issue in free flight.
@lsoderman, thanks for sharing this !
May I suggest a title update to reflect the current status, for example “Cessna 408 Sky Courrier : FLC Ovbershoot in Free Flight” ?
CHeers,
K.