I'm not sure about the V20, as I've only listened to headphones above 50 ohms on them, but it's simply a matter of IEMs being detected as low impedance and then not being supplied enough power to drive them. There's a way around it though. You can get a short 3.5 mm male to female cable. You need to first plug it into your phone so it's detected as an Aux source, and then you plug in your IEMs If you plug the IEMs into the cable first, it won't change the amp level. I do this with my AKG N40 IEMs, and they sound amazing. Now, not only is volume at 70 ear-damaging loud, but lower volume levels seem to be fuller across the board.
Something like this: https://www.amazon.com/LEMENG-Female-Headset-Extension-Cable-2Pack/dp/B01LRXP43G/ref=sr_1_8?ie=UTF8&qid=1543568122&sr=8-8&keywords=1ft+3.5mm+male+to+female
But I honestly haven't tried the second option...and I'm not sure what it would do differently, or even be harmful for the IEMs.
Hey, wanted to chime in on this since I've had a lot of experience with the last 3 V-series phones. Because the phone isn't triggered into high impedance mode (50ohms and above) is WHY your IEMs are so quiet. The sub 50ohm headphones get detected, and then the V30 and V40 lowers the amp power. I'm not sure why they do this, but to get around it I got a short and cheap 3.5mm male to female output so that it can drive my IEMs way better. Now, it thinks it's in aux mode. However, if you do this, you'll have to first connect the cable into the phone, and then your IEMs into the cable. Not both at the same time. Now, not only is it way more than enough volume for my IEMs, it sounds fuller at all volumes.
Something like this: https://www.amazon.com/LEMENG-Female-Headset-Extension-Cable-2Pack/dp/B01LRXP43G/ref=sr_1_8?ie=UTF8&qid=1543568122&sr=8-8&keywords=1ft+3.5mm+male+to+female
But I honestly haven't tried the second option...and I'm not sure what it would do differently, or even be harmful for the IEMs.