build-common-theme.xml, <target name="compile">, EVERY <copy> task needs

preservelastmodified="true"

see also com.liferay.portal.tools.SassToCssBuilder._parseSassFile(SassToCssBuilder.java:209)
if (file.lastModified() == cacheFile.lastModified()) {
return false;
}

As a side note here, in order not to open a too similar bug report
<target name="compile"> should depend on clean, or old files from _diffs will not be cleaned like so:
<target name="compile" depends="clean">

If someone can point me out to how to fix everything myself, I'd be pleased to do it.