OK - so whatever obscure epub bug I am trying to trace, it won't be connected to whatever a component file or files is/are called html or xhtml.
That's good I think, One less suspect on the list
and sigil has always used the source file type, when asked to insert a split into a file via the toolbar icon it woud never split , say ,
sample.xhtml into
sample.xhtml
sample_Section_1.html
if I'm seeing that then some other program has been at work ?