I have bought a few canisters of Tetenal C-41 fixer replenisher concentrate a while back. It works well as fixer concentrate, but the resulting pH is a bit above 7, so it has some Ammonia smell until I add a few dashes of Acetic Acid.
These replenisher concentrates typically produce a slightly higher pH than 6.5, since fixer working solution gets contaminated with acidic bleach over time. There's a non-trivial chance, that your bottles of "Flexicolor fixer concentrate" also started their life as "Flexicolor fixer replenisher concentrates". If you notice some Ammonia smell, add some acid to bring pH down to neutral. Rapid fixers are very close to odorless when they have the right pH.