Ivar found that there's a bug with ArrayDataSet.append, where it assumes that because the two datasets have the same type (double, float, long, etc) that the DEPEND_0 will also have the same type. This is of course not correct, and needs to be fixed.
This script demos:
https://github.com/autoplot/dev/blob/master/demos/2022/20220513/demoEpochBackBug.jy
This is fixed.
Interestingly Sadie ran into this bug in Dec 2021. I believe her issue was fixed then (and no ticket made), but now it's broken again. I've fixed that again, too. I think the problem was that there's both ArrayDataSet and BufferDataSet which needed similar updates.