I'm running macOS 10.14.2 on a 2018 Mac mini. I have an Akitio enclosure, connected to the Mac via a Thunderbolt 2<->3 adapter. At random, one of the drives in the enclosure becomes read-only (the other drives are fine). I'm not really sure where to begin troubleshooting this. This also happened on my 2013 Mac Pro running High Sierra.
I assume you've already done this, but I would check my connectors. Additionally, the one drive may not be getting as much power and/or can be beginning to fail. If you have a way to test it externally on another device, I think we would learn a lot more about the cause. I hope this provides some helpful ideas/thoughts. Keep us posted!