If the name of an all day event is too long in a sinle cell then their is overflow of text outside the event block. It appears behind and below the event.
There was a bug in the default CSS theme that caused the text to overflow.
It's now fixed in the latest sandbox build (2018.3.3356):
You can also generate a custom theme using the theme designer (https://themes.daypilot.org). This tool will generate a correct theme.
Note: I'm removing the "lite" tag since all-day events are only available in the Pro version.