I've been doing some testing. The plot thickens:
This behavior happens in either dual KX 155B units, or TDS GTN750/650 when in VLOC mode.
It happens everywhere, not just Alaska.
The same behavior happens in the Duke, though it changes from NAV to HDG mode. This behavior is across models, which we know from the Starship thread.
When in GPS mode, everything works fine, as I'd expect.
Here's where it gets interesting. I use SPAD for all of my planes. For a test, I restarted the sim, spawned the 208 on the runway, did NOT launch Spad but instead "moused it" for everything. No issues whatsoever, stayed in NAV tracking a VOR, departing KPHX 07L, and simply tracking the nearby PXR VOR to the east for 45+ miles.
Turned around back to PXR, started Spad, briefly switched from NAV to HDG back to NAV just for kicks, and a couple of minutes later, ROL mode.
In SPAD, I've tried both the standard "K:AP_xxx" events as well as the "H:KAP140_PUSH_xxx" commands for NAV/HDG/etc., no difference: I get ROL mode using each.
If I start a test flight using SPAD, eventually get the NAV->ROL, and I turn off SPAD, the NAV->ROL issue still occurs.
It seems that simply starting SPAD (with my rather extensive profile) correlates with this behavior.
I'll look into this some more and report back, perhaps starting with an empty Spad profile and incrementally adding things.
(In the meantime, I also might write some Spad code to detect when it goes from NAV to ROL and have it go back to NAV, though that might have some unintended side effects)