I don’t know if this is related or a separate bug, but I am also having this problem with the `+` operator and character classes. Very similar test case:
CREATE TABLE foobar (x text);
INSERT INTO foobar VALUES ('Trying +1/+1'),('Also doing +20/+20');
CREATE EXTENSION pg_trgm;
CREATE INDEX ON foobar USING gin (x gin_trgm_ops);
SET enable_seqscan TO on;
SELECT * FROM foobar WHERE x ~ '\+\d+\/\+\d+'; -- Returns both rows
SET enable_seqscan TO off;
SELECT * FROM foobar WHERE x ~ '\+\d+\/\+\d+'; -- Returns 0 rows