Yeah, this is a new change introduced with Storm League this season. However, I do not think it was included in the patch notes. This is why I wanted to respond here and get some official confirmation that this is the intended functionality out into the world 
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Random Heroes are not ideal for a competitive mode as it can put your team at a disadvantage if you get a Hero that is sub-optimal for the map or current team compositions. This change was made to address feedback that this can be a bad experience.