There is a problem with positioning a footnote on a page.
"XEP can place footnotes inside single columns in multi-column layouts, rather than a page-wide area (as prescribed by the spec). This behaviour is controlled by an option, PAGEWIDE_FOOTNOTES="false"."
This option has been set in the xep.xml file, and indeed the footnotes then are placed inside the single columns, but in some cases the footnotes no longer appear at the bottom of the page, but elsewhere in the layout. This occurs when the layout changes from multi-column to single column during the flow. The footnote is then placed just before where the single column starts.
Anyone knows how to solve this?
Placing footnotes inside single columns is a RenderX extension to the specification. This only makes sense when the number of columns on a page is constant, and no flow-sections or blocks with span="all" appear on the page. The latter is exactly your case.
Consider if the block with span="all" contains more text than just a subchapter title and does not fit the space left on the page. It will meet the page break and continue on the following page. So the bottom of the page where the block with span="all" starts is rendered in sinle-column layout, and narrow column-wide footnote in one column would make it look bad.
There can also be a design which uses rx:flow-setion to switch from 2-column to 3-column layout and then back from 3- to 2-columns. All on the same page. Where should the formatter place the footnoes in this case?
Anyway, XEP flushes all the column-wide footnotes when the flow-section ends, placing them before the next flow-section (or a block with span="all"), and there is nothing one can do about it.
The problem arises with headings, whose marker starts with m: \ms, \ms1, \ms2. \mr, etc. When editing those styles (File/Styles/Edit, then File/Style/Properties), switch off the feature: Spans the 2 columns, and the problem is gone.