Reminders and Calendar is inaccurately suggesting date when weekend is selected from suggestions or naturla language

I have set my week to start from Monday, and all my language/region settings are accurate. 

Tried: re-installing, resetting, changing region/language.

Problem: If I write 'weekend', it considers Saturday, which is correct. If I write every weekend, it considers the following Sunday and Monday for some reason. If I set my week to start from Sunday, then every weekend gets counted as Saturday and Sunday. The problem affects both macOS and iPhone.

If I manually select Saturday and Sunday, then it chooses the weekend option itself (renames it), but it doesn't work when I use natural language or the suggestions above the keyboard. Even the suggestion strip and the suggested dates on the buttons in the app, above the keyboard, are incorrect. 

The same thing happens with Apple Calendar. I have spoken to Apple Support, and they obviously have no clue about this. They said they will escalate the issue, but it's likely lost somewhere now.

Anything on this would be helpful; one of the significant things stopping me from leaving Todoist.