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I just had the same situation happen to me on a repair. As Daniel mentioned too many variables, even if the same person mixed it for just a repair vs. the whole floor. Temperature, humidity, amount of water used to mix, dry time, amount of wash water, etc.
My best suggestion is to strip the sealer used, and color seal the floor grout.
I'll warn you that your grout may crack again. Are you sure you don't have any loose tiles? That's the main reason for grout cracking in the first place.