Index corruption revealed after upgrade to 11.17, could date back to at least 11.12

Dear community,

Right after upgrading our postgres servers from 11.15 to 11.17 we
started to encounter problems with data. Namely, when the query hit
the index, it returned a single row; when the query hit a relation
directly, it returned more than one row. Attempt to REINDEX revealed
the underlying data had duplicates (unique index reindexing failed).

Version facts:
we started out with 11.12
jan 2022 upgraded to 11.14
mar 2022 to 11.15
oct 2022 to 11.17

We are not sure when this corruption actually happened. Could it be
related to the indexing bugs reported in
https://www.postgresql.org/docs/release/11.14/? And the condition only
became known to us after 11.17 rollout which can perhaps be explained
by the following: while 11.17 does not have any outstanding index
related fixes, then https://www.postgresql.org/docs/release/11.15/
mentions fix for index-only scans and so does
https://www.postgresql.org/docs/release/11.16/.

The bottom line is we would like to understand if the index corruption
and its manifestation is explained by the above release fixes or is
there something else that should be investigated further here with the
help from the community.

With best regards,
--
Kristjan Mustkivi

Email: kristjan(dot)mustkivi(at)gmail(dot)com