This has been an intermittent issue for a long time. I have AP1 and AP2. Both with software edition: 2017.34 2448cfc
With AP1, over the years it was bad, then fixed, then came back, then fixed, etc....
With AP2, over the year (singular) it has been present*, fixed, then came back, etc...
I've found that the greatest variability is the light level on the lines.
If the lines (especially the left line) are highly visible, the car will recognize the line and follow the highway and NOT exit. The exception is the route where the exit is "straight" and the highway curves to the left (In the US)... then the car understandably starts to exit.
On my daily commute (traveling west at the end of the day) with AP1 there is an exit which the car will dive for if I am coming home late and the sun is lower. At other times, it will follow the highway. This also happens with AP2 when I test/get to drive it. I've noticed that the left blue line will disappear, and the right line will stay blue--then the car will exit (daily commute, I'm so used to it I can actually spend the effort to look at my dash).
With wet highway conditions (not actively raining) I've experienced the same issue, but more often and with normally "safe" exits.
Sometimes the car will start to dive, and then bring itself back to the highway lane. I have certain exits on my commute where this will happen. I've just gotten to know these exits and just remain more vigilant.
The obvious way to remedy this "exiting" problem is to drive in the left lane (bad if you're not a speed demon, like me), or if you have the luxury of 3 lanes, stay in the middle.
*not as bad as AP1 initially btw
PS--I use Autopilot (TACC & Autosteer) close to 100% each way on my commutes. And pretty much any time I have highway driving... even short bits for 1 mile or so.