Why would a rice seem "non-absorbent"?
Based on your edit, my best guess is that this particular rice has a really high amount of surface starch, which gelatinized when you first tried to cook it (liquid from the mushrooms might have been enough to get this result).  That would have created a moisture-resistant layer around the grains that inhibited moisture from soaking in further.  Your soak would have washed that away and allowed the rice to properly absorb the broth.

That's definitely weird - probably a byproduct of how this particular brand gets processed.  A further experiment worth trying would be to briefly rinse off the grains in a colander for your next batch.  That would also get rid of surface starch without quite the same effect as a soak.  If they work after even a brief rinse, I'd consider that pretty good evidence for this theory.

Another thing you could try might be sauteeing the rice in a completely separate pan from the mushrooms, so that they're exposed only to oil and the surface starch gets toasted instead of gelatinized.  Maybe not quite as desirable from a flavor perspective, and of course you'd have to clean another pan, but it's worth testing if you really want to figure out the root of the problem.