Week display, first day of the week changed to Monday – but the contents remain

Problem/Motivation

I would like to upgrade our Drupal 7 instance on which you can book tennis courts.
The booking is implemented with Calendar + Registration. It has been running without errors for 9 years.
A node is imported for every free half hour, which can then be booked via registration. We have 3 tennis courts indoors and 9 outdoor courts.
For the indoors I have 3 week displays (separate for each court).
Outdoors the day display is available for all 9 courts (slightly modified in the calendar template for the day view).

I am currently working on the week display for the tennis hall under Drupal 10.4.2.
My problem arises when I switch to Monday as the first day of the week: only the header of the calendar is changed (the "days" line) - but the bookable tennis courts do not move with it.
This means that "Monday" is displayed in the header of the first column, but below that are still the tennis court hours for Sunday.
I have tried it with different date fields. But always the same result.

Steps to reproduce

1. Create any date field
2. Create many nodes for all days of the week (each node has exactly one date)
3. Create a calendar week view
4. Set the first day of the week to "Monday" (under admin/config/regional/settings)
5. --> View calendar: The nodes are in the wrong column

Thanks for any help