Here's my fixer story.
When I started with home lab processing some of years ago the first bottle of fixer was OK (don't remeber which), it lasted about 2 years with no probs.
Then I was given a half filled 20+ years old Tetenal Superfix in its plastic bottle and it was clear, smelled fresh and nice and did the job perfectly. Wow! Thumbs up for this Methusalem!
When this bottle was used up, next fixer was branded Adofix from Adox. After about 1 year the concentrate became a stinky, cloudy disgusting sludge and I dunped it. I read of others having the same prob with this fixer. All thumbs down!
Then I bought a 3 liter canister of noname rapid fixer and it first worked nicely. But soon the negatives got billions of white spots, only the first film fixed in working solution was OK. It took me some frustrating time to see what the problem was. I dumped more than 1 Liter of the remaining concentrate. Thumbs down again!
Now I'm using Agfa Fix-Ag, and it seems to be of very good quality that solved the spots-problem immediately. The working solution keeps for at least 1 year and the concentrate is still like new after maybe 2 years. Thumbs up again :-) My negs are finally clear and spot-free again. Phew, I took a deep breath....... Besides the decades old Tetenal the best fixer so far.
Conclusion: fixer obviously is not as trivial as it seems to or could be. As we have seen here, also premium branded fixer can be a pita. Recipes, raw chemicals, quality control or what ever can be inferiour. We should name the bad working, bad keeping products like we do it here and write to the producers.