This looks like a live bug in pg_trgm to me.  I can strip away much of the stuff and still reproduce it with this simple test case:

    create table foobar (x text);
    insert into foobar values ('eldrazi scion'),('eldrazi scio');
    create extension pg_trgm ;
    create index  on foobar using gin (x gin_trgm_ops);
    select * from foobar where x  ~ 'eldrazi (?!scion)';  -- returns 1 row
    set enable_seqscan TO off;
    select * from foobar where x  ~ 'eldrazi (?!scion)'; -- returns 0 rows

Note that a similar bug was fixed recently (in yet-to-be-released code, commit 16500d2278ab3dd), but that fix does not fix this bug.

I've reported this bug myself, as #14623