The 42 day average shown in PowerCenter is using the unweighted average whereas the 42 day average shown in the mobile app is using the weighted moving average.
The RSB in PowerCenter is still calculated using the exponential and will be the same RSB that is shown in the mobile app, the change only applies to the averaging that is shown in the My Training Chart. The averaging method for all metrics (RSS, distance, duration) in the My Training Chart is unified, all use normal averaging.
The reason why the weighted average is shown the mobile app but not shown in PowerCenter is due to the combination of the Coach’s View feature (multi-user support) and the option to select custom time period ranges within the My Training chart on PowerCenter. PowerCenter My Training Chart uses normal average instead of weighted average to make the flow fast enough to support multiple users.