The fuse will do its job, but the margin will be much lower than a .6A fuse, meaning that it will blow quicker when the current goes above 0.6A. At 10$ a pop, it could be an expensive fuse solution.
I'm not an expert, but it seems that the Time-Current charts are usually derived from 50Hz RMS currents (10% accuracy), with lower margins for DC currents.
As far as IEC CAT rating is concerned, I'm not sure if a lower fuse rating is allowed under the current regulations. I haven't heard of any other manufacturers using lower rated fuses in their meters yet.
The higher resistance of the 0.4A fuse (~ 1.2Ohm) should also make the burden voltage slightly worse on the mA scale.
It would be interesting to see how long it takes for the 0.4A fuse to blow, when a 0.6A current is flowing through it.
The fuse rated current (400mA) is the maximum continuous current allowed, without rupturing the fuse, guaranteed by the manufacturer. 600mA may be too high for continuous measurement and falls outside the fuse certified rating anyway.