My prints go brown in the fixer too, but I never observed bleaching, just color changed. The tones get slightly darker, have reddish maroon like color after complete dry down.
I use a hair dryer to hasten it for test strips.
Exposure can be misleading, when I started experimenting salt print I thought 3-4 minutes exposure was ok, then I thought 7 is good, but later I found out 12 min was spot on.
If you are using film, the exposures might be even longer.
Well the times can differ for everyone's workflow but reaching the darkest tone in the print during exposure, not necessarily means it is exposed enough for salt printing IMHO.