I was initially having a major PITA problem with my 945 and the 2 CIQ fields, because somehow the watch thought I had more than 2 fields in use. I could set it up, and it would work for a run, then it would lose my Core field the next workout. To fix it, I removed both CIQ fields, rebooted the watch, added them both back, and it has been fine since.

Edit: I remembered one more thing that is a little different for both of these devices. Both the Stryd and Core mobile apps have an input field where you enter the device's ANT+ ID. I never paired my Core to the 945 - I just entered its ANT+ in the phone UI. For the Stryd, I did pair it and I also added its ANT+ ID through the app. Maybe this is related - how did you connect & pair your devices to the watch?