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