Yes, this can be normal with some third-party lenses. A likely explanation is that the lens reports itself to the camera as being a little faster than it really is. One reason is autofocus compatibility: some cameras limit or disable autofocus once the maximum aperture gets smaller than a certain value, so reporting f/5.6 instead of the actual f/6.3 can keep AF available.

In practice, the lens is still physically only as bright as it really is. The reported f/5.6 is mainly a communication/metadata issue between the lens and camera, not evidence that the lens has somehow become faster at 300mm.

The exposure difference between f/5.6 and f/6.3 is only about one-third of a stop, so any exposure error is usually minor. The main effect is that autofocus may work when the camera would otherwise refuse it, though performance may be slower or less reliable.

So yes: the behavior is plausible and generally not a sign that anything is broken.