@olgatsyba -
As far as I know, the only way to create sticky notes that way would be bulk mode where the sticky note content for multiple notes is typed in a separate dialog box and they are created only after the content has been entered.
Other than that, someone might create them in a different Miro board and copy & paste them into the common board at a synchronized time.
Kiron
@olgatsyba - As @Kiron Bondale mentions, it sounds like your team member was using the Sticky Note Bulk Mode:
Bulk mode
To add a bunch of stickies in a row, click the sticky note icon, choose Bulk mode and start typing away.Hit Enter to separate ideas. The limit of symbols depends on the font size. Click Done, when you finish, and your ideas will be added to the board as stickies.
Adding stickies with a bulk mode