@outputclass not present in MERGED output when using maps

[MiramoPDF]

Good day all,

(This is reposted from the dita-ot users group)

I'm hoping someone could advise me on a DITA-OT issue which keeps cropping up, where @outputclass values are not passed through to the DITA-OT topic merged file, and hence are not available for use in custom plugins.

Attached is a bookmap which has @outputclass values applied both to the elements:

<chapter href="submap2.ditamap" format="ditamap" outputclass="chapterNumberStart=4"/>

and to the referenced ditamaps (this is submap2.ditamap):

<map outputclass="chapterNumberStart=4">

Neither @outputclass value is passed through to the topicmerged file. The DITA-OT 4.x topicmerged file produced when publishing the above map using Oxygen 26.1 (XSL-FO/PDF2) is attached to illustrate this.

I believe this could be addressed through a DITA-OT extension point, but have no idea where to start with this - is this something any of you could help me with?

Many thanks in advance for any advice.

Best wishes!
test-bookmap.zip
test-bookmap_MERGED.txt