We recently upgraded to v7.4.0 from 7.3.5 and one of our stacked graphs is no longer showing correctly. It appears to be related to missing data points for some time periods for some series.!
Here is the non-stacked version.
Here is what it now looks like when the graph is stacked.
Changing the null value setting to "connected, “null”, or “null as zero” makes no difference.