In standard, you can find the all links where the text is coming from.
Go to OLME-Purchase Requisition-Texts for Purchase Requisitions-Define Copying Rules
Here, select Item text and click on Text linkages. You will find the linkage from where the text is coming.
Second Part : Its quite not possible in standard, Check is there any development which is removing the text when you press save button.
Third Part: Yes. In standard, material PO text will come when you will use material in PR. Other than the field will show in your text line, but it will show blank.
The green tick mark shows when a text is successfully copied from any source.