I closed the GlassWire dekstop application and services and restarted them. GlassWire now shows the correct Graph and Usage details since the restart but the lost data has not been recovered.
Restarting GlassWire also caused/revealed another issue. The bandwidth overage settings became turned off. As you can see in the screen shot I’d made the changes in the current session. I’ve now re-enabled them after checking the weekly warning worked: