I had 49 hours last night - then I flew a 2.5 hour flight to get the 50 hour achievement, and I had 48 hours after the 2.5 hour flight.
( I’ve lost an hour or two due to CTD before, but this wasn’t the case. Successful flight, followed by parking and shutdown. )
Also, I’ve had zero luck in getting the following achievements ( I’ve pretty much given up on achievements for now )
Wildlife - flew 3 times over three different areas / animals - made multiple passes each time with smart cam
ILS - have made 4-5 different approaches/landings with LOC/GS tuned - no GPS
Full Service - have 83% complete, but it won’t tick-off the full truck after using it 3-4 times
In general - Achievements really need an (optional) in-game popup when they’re competed successfully, so you don’t have to quit the flight, then check your profile to see if you got it or not.