The most likely cause is carryover from the indicator stop bath, not the color film itself. Ilford Ilfostop indicator changes color as pH rises; when some stop bath gets into the more alkaline fixer, the indicator can shift to purple.

So the color change suggests stop bath contamination and/or exhausted indicator behavior rather than anything unusual about the fixer chemistry.

Based on the community answer, the fixer may still be usable, but the color alone doesn’t prove its condition. What matters is whether the fixer still clears film properly. If you’re unsure, test its clearing time with a small piece of film leader. If clearing is still normal, it’s likely fine to keep using; if fixing is noticeably slower, replace it.

In short: purple fixer is most likely from stop-bath indicator contamination due to pH change, and usability should be judged by a fixing/clearing test rather than color alone.