Label autocomplete not populating in label edit dialog for pages in one particular space

I'm seeing the behaviour described in CONF-29989, but in one particular space.

I have confirmed with some random testing that 6 different pages in one particular space (including a new page) do not show the auto-complete for labels, while 1 page in each of 6 other spaces do show the auto-complete labels (including labels only found in the one 'broken' space).

Unfortunately, I do not know yet how to 'break' a space. I tried the test on CONF-14279 and it did not break the new space w.r.t. the label autocomplete dialog

I would have raised this at https://support.atlassian.com/customer/servicedesk-portal, but the site is down.

What can I do to help further diagnose the reproducibility of this issue?