The Query is doing exactly what is should be doing

as Ron pointed out 29/06/2020 is a Monday while 30/06/2020 is a Tuesday

What you're asking it to do is:

"Tell me the corresponding Monday date of the date I'll give you"

So 29/06 (Monday) 30/06 (Tuesday) 01/07 (Wednesday) 02/07 (Thursday) 03/07 (Friday) 04/07 (Saturday) 05/07 (Sunday)

Will all return 29/06/2020

So the question is rather, what do you actually want to do ?