Dropdown list clearing after creating new item

Hi

I want to make a new item list (project list) with field taken from other list.  (request list)

After creating the new project I want the selected items to be gone from dropdown list but not from the request list. (I want workers to look up into request)

Example:

Create Project 1:

Attach request:

• Request A (selected)
• Request B (selected)
• Request C

Create Project 2:

Attach request:

• Request C

(A and B are gone because they were selected in Project 1)

I tried to copy, delate items in MS flow but that didn't` work well... Do you have any ideas?