Saxon

As regards the "aside", it's a feature. Processing instructions and comments in the stylesheet are stripped, they don't act as PI or comment constructors as in XQuery. Poor design choice perhaps, but it's what the spec says.

The output I'm seeing is:

PIs:  1
Text: style="background-color: red;"
style background-color: red; 
Text: 
map: Error XTDE0450 while evaluating xsl:message at line 52 of file:/Users/mike/bugs/2020/4538-Tovey-Walsh/test.xsl: Cannot add a map to an XDM node tree
pimap: Error XTDE0450 while evaluating xsl:message at line 46 of file:/Users/mike/bugs/2020/4538-Tovey-Walsh/test.xsl: Cannot add a map to an XDM node tree
Error XTTE0780 while evaluating xsl:message at line 17 of file:/Users/mike/bugs/2020/4538-Tovey-Walsh/test.xsl: An empty sequence is not allowed as the result of a call to fp:pi-attributes#2

I suspect it's all to do with error handling in xsl:message. xsl:message is defined to produce an XML document node, which is a bit unfortunate when you want to report on the content of maps.

I changed the "map" and "pimap" messages to use, e.g.

<xsl:message select="('map:',serialize($map, map{'method':'adaptive'}))"/>

but the final error remains.

I've now changed the xsl:message on line 17 to xsl:value-of and the main effect is that I get better diagnostics:

Type error on line 52 column 81 of test.xsl:
  XTTE0780  An empty sequence is not allowed as the result of a call to fp:pi-attributes#2
at function fp:pi-attributes on line 40 of test.xsl:
     Focus: absent
     Local variables
        $pis = <?db...?>
        $pimap = map{}
        $map = map{"style":"background-color: red;"}
     invoked by function call at file:/Users/mike/bugs/2020/4538-Tovey-Walsh/test.xsl#26
at function f:pi-attributes on line 20 of test.xsl:
     Focus: absent
     Local variables
        $pis = <?db...?>
     invoked by function call at file:/Users/mike/bugs/2020/4538-Tovey-Walsh/test.xsl#17
  In template rule with match="/" on line 16 of test.xsl
An empty sequence is not allowed as the result of a call to fp:pi-attributes#2

though sadly this gets me no closer to a solution...

Ah. Interesting. It never occurred to me that the problem was in the message. I put them in because I was just developing the code and wanted to see what was going on. I'll rewrite the message to avoid trying to serialize the map.

I think the answer here lies in the fact you use nested xsl:message. It's not clear what the output order, as there are two "3a".

fp:pi-attributes() returns a map.

So line 53

<xsl:apply-templates select="fp:pi-attributes(subsequence($pis, 2), $map)"/>

is applying templates to a map.

You don't have a template rule that matches maps, and the built in template rule for maps returns an empty sequence. The apply-templates result is used as the function result, so the function attempts to return an empty sequence, contrary to its declaration.

Have I missed something?

Nope. That'd be the "complete stupidity" that I alluded to at the beginning. Apologies for the noise.