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Do you have the XFES add-on installed?
I don't use the Similar Threads add-on, but on my site (and the site I sold) they both have XFES installed and give you a similar threads ability. On my Astronomy site it is enhanced by ElasticSearch Essentials and Search Improvements add-ons.
If you notice... the first example displays differently than the second. The first uses the additional add-ons, the latter one only the XFES add-on from XenForo.
I have found every now and then when an add-on gets wonky, doing a master rebuild of the site can help. I've even had issues with XenForo that this resolved and showed errors that were not being reported prior to it.
I usually try to trouble shoot stuff myself as much as possible as I don't really like dealing with anyone over with XenForo (the company).
I do submit some tickets for documentation purposes for certain pursuits.
It's fascinating and especially useful for sites like this, which have over a decade of conversation. With some of the evergreen topics in community management, you actually find similar threads posted years ago!