Weekly_Start or Weekly seasonality appears to be changing in the forecasted values after there is no Actuals data with the Prophet algorithm

[zender33]

Describe the bug
I've created a daily prediction model.
The data I am trying to predict should have only data for the weekdays Mon - Fri.

I noticed that the forecasts for the period where there are no actuals, there are values for Sunday.

The data also has very strong weekly seasonality.

When reviewing the forecasted values for the period where I have actuals data, they have very similar seasonality

However, once there is no actuals data, the forecast seasonality appears this way:

It appears that the values are distributed correctly as proportion, however, the Weekly_Start is suddenly set to 0.

To Reproduce
I am using the following arguments

and also added Holidays.

Although the default Weekly_Start should be 1 by default, I've added the parameter just in case, however, it did not change anything.

Expected behavior
I expected that the future prediction should be predicated for the weekdays only and the Weekly_Start should be kept.

Environment:

• OS Windows Server 2012 R2
• Qlik Sense Enterprise June 2020
• SSE Version: 8.1