I haven't seen anyone mention/confirm here yet, but just in case anyone was wondering, I just checked out the superior exotic rewards for this Cosmic Threat event and confirmed that the slots you get gear for are completely random, instead of being determined by which slot has your lowest ever PL gear. This applies to both the Meta and Minor objectives.

So now that we've had all 5 events, we have the following:

• Tachyon Anomaly - Meta objective only drops superior gear matching the lowest PL slot of the hero completing it; Minor objectives also drop superior gear depending on the lowest PL slot of the hero completing them
• Rooskaya Protocols - Falls into neither category as the Meta objective gives 1 defense and 1 heroic piece and the Minor objectives give 1 melee and 1 ranged piece (all superior)
• Corrupted Vibranium - Random pieces for both Meta and Minor objectives (only the Meta objective gives superior gear for the main 4 slots; Minor only gives superior minor artifacts)
• Red Room - Meta objective only drops superior gear matching the lowest PL slot of the hero completing it; Minor objectives do NOT drop superior gear
• Cosmic Threat - Random pieces for both Meta and Minor objectives (both give superior gear).