There is useful information regarding fixer use in the Ilford Rapid Fixer document. It advocates 2-bath fixing on page 3 column 1.
Read the information on page 4 beginning with the bold heading
Silver Concentration and the following paragraphs to the bottom of the page (2nd column). In particular, read the information beginning with
“Prepare the testing solution by dissolving 2g of sodium sulphide in 125ml of water.”
https://www.ilfordphoto.com/amfile/file/download/file/1833/product/711/
From a Google search:
“Key points about yellow stains from rapid fixer:
Under-fixing:
The primary reason for yellow stains is not fixing the print long enough in the fixer, leaving silver halide crystals that can oxidize and turn yellow over time.
Insufficient washing:
Even with proper fixing time, inadequate rinsing after fixing can still leave behind fixer residue causing yellowing.
Old fixer:
Using old or overly diluted fixer can also lead to poor fixing and yellow stains.”