I’ll assume you meant Houndmaster Shaw (“Your other minions have Rush”) and not Houndmaster (“Battlecry: Give a friendly Beast +2/+2 and Taunt”).
Normally, when a minion transforms, the new minion is considered to be a new, just summoned minion. Which means that one would expect it to have summoning sickness, and that Rush or Charge would partially or completely exempt it from summoning sickness.
Based on what you describe, it seems as if Infectious Sporeling works differently. As if it changes the target minion without making it a new summon, but instead retaining some of the characteristics of the old minion. If that is indeed the case, then the consequence would be that, if I somehow manage to force an opponent Sporeling to attack my minion on my turn, before my minion attacked, it should still be able to attack.
This is definitely an interesting bug report. I first thought that somehow the Houndmaster Shaw effect failed to be applied to the fresh summoned Sporeling. But in that case, you would have seen the “Give that minion a turn to get ready” error when trying to attack, not “That minion already attacked”.
Hard to tell whether this is intentional or not. It seems like a weird and inconsistent choice to me, so perhaps there is indeed an oversight in the code somewhere that should be fixed.