As it stands today, if you are in a Rift with Getaway Artist equipped and an Arc Soul has been given to you, using Getaway Artist does not spawn a Sentient Arc Soul. Similarly, if I have an Arc Soul as the result of using a Rift and attempt to use Getaway Artist, it simply refreshes the timer for the Arc Soul and does not spawn a Sentient Arc Soul
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Sentient arc soul and arc soul while amplified are the exact same thing. When you consume your Grenade while already having an arc soul from your rift, you’ll be amplified and your arc soul will be buffed exactly the same as a sentient arc soul. Bungie updated it last season.