Failure to create GiST on ltree column

I am getting random failures in `CREATE INDEX USING gist` over ltree column while performing pg_restore.
I get either
    ERROR:  stack depth limit exceeded
or

    ERROR:  failed to add item to index page

Thing is — if I retry index creation manually, I get it successfully built in ~50% of the cases.

I would like to find out what's the real cause here, but I am not sure how to do it.
If anybody could provide some guidance, I am open to investigate this case.

I'm on PostgreSQL 11.8 (Debian 11.8-1.pgdg90+1), debugging symbols installed.
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