I don't know what causes it, as it happens randomly. 1st time I was on break at work, 2nd time I was visiting family for the holidays.
As you can see from the video, the SIM card is acting like it randomly disconnects/reconnects rapidly. Throwing it in and out of Airplane mode doesn't help, rebooting doesn't help; the only solution was taking out the SIM card, blowing inside the tray, and reinserting it.
The 2nd time it happened, I didn't have a SIM card ejector tool, but it seemed like it randomly went away.
I've had this unit replaced last week as it had a green tint defect, but my previous unit experienced this bug also.
I don't think it's an issue with the SIM card, as my OnePlus 6 reads my SIM card just fine for the previous year that I've used it.
Video of what happens: https://youtu.be/a1j-6o5ChLM