I had some print dilution TF4 that sat in a jug for a few years
that I forgot about and the solution turned a weird darker color.
Never seen this with fixer.
Doubt it's related to your issue but how old is the TF4 you're using?
Could your safelight have been compromised since your previous successful printing sessions.
If your safelite is closer to the fixing tray and is unsafe it could explain.