This is because the newest Watch OS8.1 is not compatible with Dexcom's Follow app. I bought Apple Watches for my wife and I to help us manage our daughter's Type 1. I guess I forgot to turn off automatic updates and when my watch updated to 8.1 the complication on the watch went blank. Now there's nothing that we can do but wait for Dexcom to catch up with updates to their Follow app. Dexcom's site has a compatibility tool that you can use to check for phone and watch OS updates.
This is a pretty big problem: Dexcom can't start working to make sure that their apps work with Apple's updated OS until they are released and once they are, it takes a while. Dexcom is always a few updates behind. Apple doesn't make older versions of OS updates available to install. That pretty much means that you either NEVER update any Apple devices OR you have to roll the dice and hope they are compatible.
This would be a SERIOUS problem if/when an Apple OS update breaks the Dexcom Follow app on a phone or the Dexcom G6 app on the phone belonging to the person that has diabetes and wears the Dexcom sensor. They'd have to go back to using the Dexcom receiver until Dexcom "fixes" their app and pushes out an update.