Cheers @notbetting — had a look and it wasn't that, but good advice nonetheless. Turned out to be some unfinished functionality I'd staged for a future release but not fully removed— seems that may have tripped a 5.6 pathway. Removed it, rebuilt and resubmitted. Fingers crossed.

Following up on this — it's now over a month. The submission (b4e5d54c-de30-4003-9f3c-232e366d01c4, version 1.1.0 build 7) has been in "Waiting for Review" since 16 June and has never moved to In Review. Still no Resolution Center message and nothing action-required on my side. I've also opened support case 102923511308 (reply received but only a generic acknowledgement, no status or timeline), and submitted an expedited review request — which I've had no acknowledgement of at all, not even a receipt or decline. This is an update to a live app, and the release contains accessibility fixes (increased contrast, larger tap targets, Dynamic Type support), so users installing today are getting the version without them. Could someone confirm why it hasn't entered review, or flag if anything is needed from me? @Apple