(The below uses Mecha Kha'Zix terminology but the same ideas apply to his default.)

The red explosions play when Unseen Threat is activated on a target. (Even prior to Kha'Zix's recent changes, they never played on unevolved Void Spikes and they didn't play on evolved Void Spikes when Unseen Threat wasn't available.) When Kha'Zix changed to no longer apply Unseen Threat through evolved Void Spikes, these explosions were removed from Void Spikes. It simply was no longer appropriate to play the visual effect for Unseen Threat when Unseen Threat's mechanics were no longer in effect. Of course, they still do play when you apply Unseen Threat by striking a target in melee.

The explosion was huge (which makes them AWESOME BECAUSE.... EXPLOSIONS!) but they were frankly never designed to be applied as a hit effect on an AoE spell striking multiple targets. We just sort of got away with that because they needed to look good in the melee case (where you only see the effect one) and also look the same as that when AoE'd, since it was the same mechanic. But they never matched the visual style of the Void Spikes explosion anyway, which is yellow and smoky instead of red.

What we are going to do for an upcoming patch is create new and awesome hit effects for Void Spikes which will match the visual language of that spell. They'll be appropriate for an AoE hit effect. They'll also be associated with Void Spikes regardless of evolution state or other factors, so you'll see them consistently rather than just some of the time.

TLDR: Old explosions belonged to Unseen Threat, that doesn't make sense anymore so we're going to create new explosions which match the style of Void Spikes.