Did you accidentally change the Navigate on Autopilot settings. You can turn off auto-lane change or make it very conservative
Require Lane Change Confirmation: By
default, Navigate on Autopilot requires
your confirmation before proceeding with
a lane change (by engaging the
appropriate turn signal). However, if you
want Navigate on Autopilot to change
lanes without requiring this confirmation,
turn this setting off. When you turn the
setting off, you can specify if or how you
want to be notified of lane changes (Off,
Chime, Vibrate, or Both).
and...
Speed Based Lane Changes: Navigate on
Autopilot is designed to perform both
route-based and speed-based lane
changes. Route-based lane changes are
designed to keep you on your navigation
route (for example, moving you into an
adjacent lane to prepare for an upcoming
off-ramp) whereas speed-based lane
changes are designed to maintain a
driving speed (not to exceed your cruising
speed) that allows you to minimize the
time it takes to reach your destination (for
example, moving into an adjacent lane to
pass a vehicle in front of you).
Speedbased lanes changes are optional. You can
use this setting to disable speed-based
lane changes or to specify how
aggressively you want Navigate on
Autopilot to change lanes to achieve the
set cruising speed. The MILD setting is
more conservative about lane changes
and may result in a slightly longer driving
time whereas MAD MAX is designed to
allow you to reach your destination in the
shortest driving time possible, but will
only change lanes when safe to do so