We appreciate you clarifying this for us, Ctlimey,
What do you have set as your default calendar? You can go to Settings > Calendar > Default Calendar to check.
Does it make a difference if you choose an alternate Calendar as your default Calendar?
You can learn more about using multiple calendars here: Set up multiple calendars on iPhone - Apple Support
Does it make a difference if you disable Week View Starts On Today?
Customize your calendar on iPhone - Apple Support
Customize your calendar on iPhone
In the Calendar app , you can choose which day of the week Calendar starts with, display week numbers, choose alternate calendars (for example, to display Chinese or Hebrew dates), override the automatic time zone, and more.
We look forward to your reply.
Cheers!