One of my TuCows/Hover.com domain names became unresolvable recently that was DNS hosted with servermatrix.com.
So it appears IBM Cloud/Softlayer/ServerMatrix may be doing some zone cleanup in 2019, and you should want to double-check any domain names hosted there. Or better yet, add monitoring.
TuCows/Hover.com chat support was very helpful in determining the problem and resolving it. Bad pun, ya? 🙂