A consistent distance measurement that suddenly changes by a tenth of a mile is usually caused by one of a few things.
The most likely explanation is a change to the calibration data. Apple Watch uses a combination of GPS and motion sensors, and it periodically recalibrates its stride length estimate based on your recent walks. If your gait has changed even slightly, or if a recent walk was measured more with GPS than with the accelerometer (or vice versa), the recalibration can shift the stride length calculation slightly and affect all subsequent readings.
You can reset the calibration data to start fresh. Open the Watch app on your iPhone, go to Privacy > Motion & Fitness, and tap Reset Calibration Data. After doing this, take your usual walk with iPhone in hand so the GPS can re-establish a clean baseline.
Another thing to check is whether the watchOS 26 update changed anything in how your height is recorded, since stride estimation is tied to your height in Health. Open the Health app on your iPhone, go to your profile, and confirm that your height is entered correctly.
Finally, if the watch had an update recently it sometimes defaults to using the accelerometer more and GPS less for short activity periods. Making sure Location Services for the Workout app is set to "While Using" can ensure GPS is fully engaged for your walks.
