This is a known limitation of Extension:Labeled Section Transclusion that content from transcluded sections cannot be further transcluded. I'm not sure if that is what I am experiencing here for certain since it doesn't seem like what I would expect from that description?

On Legislation:P.A. June 2021 SS 21-1 S.143 (US-CT), I tried storing the results of <pages index="index.djvu" from="?" to="?" fromsection="XX" tosection="Y" />into the text field of the "Act_Sections" table. The resulting value was ?'"`UNIQ--pages-00000000-QINU`"'?.

I would've expected no value if these two extensions can't play nice with one another; not a strip marker. That was the case for how Wikibase and Cargo interacted with one another at least (see CGS 1-1h(c) for that test).