iOS version 12.1.1 works fine with the 3DR Solo app with ONE exception: If your GoPro's resolution is above FHD (1920×1080) at 30 FPS, then when you press the RECORD button the live video feed MIGHT go black (although the GoPro is still recording fine). There are two easy workarounds: (1) While still recording blind, exit the Solo app and restart it. The live video feed of the recording already in progress will appear. (2) Reduce your GoPro's resolution and/or frame rate until the problem goes away. Different GoPro's seem to have different thresholds.
For what it's worth, I've been flying almost every day for many months with the iOS Solo app from 3DR, and keeping my iPad version up to date, without any trouble, since FHD at 30 FPS is perfect for my current needs.