Stored procedure is neither sortable nor filterable, although commented as @sortable and @filterable
### Summary

In GraphiQL, the fields `orderBy` and `condition` are missing for the stored procedure `search_powerups(varchar)` (see below), although it is properly commented as `@sortable` and `@filterable`.

### Steps to reproduce

```bash
#!/usr/bin/env bash
# stolen from https://github.com/graphile/postgraphile/issues/1469#issuecomment-840468985
set -e
DATABASE_NAME="db_9ec0daec_e399_11eb_9008_47882ecae7df"
SCHEMA_NAME="public"

yarn add --dev postgraphile@next
dropdb --if-exists "$DATABASE_NAME"
createdb "$DATABASE_NAME"
psql -X1v ON_ERROR_STOP=1 "$DATABASE_NAME" <<HERE

create extension pg_trgm;

create table powerups
(
    id         uuid primary key default gen_random_uuid(),
    name       varchar not null,
    created_at timestamp default current_timestamp
);

create index powerups_on_fuzzy_name on powerups using gist (name gist_trgm_ops);

insert into powerups (name) values ('cooking'), ('skating'), ('swimming'), ('coding');

---

create type powerup_search_result as (
    similarity real,
    word_similarity real,
    powerup powerups
);

create or replace function search_powerups(by_name varchar)
returns setof powerup_search_result
as \$\$
    select
        similarity(by_name, p.name) as similarity,
        word_similarity(by_name, p.name) as word_similarity,
        p as powerup
    from powerups as p
\$\$ language sql stable strict;

comment on function search_powerups(varchar) is E'@filterable\n@sortable';

HERE

# Recommended options from https://www.graphile.org/postgraphile/usage-cli/
yarn postgraphile \
  --subscriptions \
  --watch \
  --dynamic-json \
  --no-setof-functions-contain-nulls \
  --no-ignore-rbac \
  --show-error-stack=json \
  --extended-errors hint,detail,errcode \
  --export-schema-graphql schema.graphql \
  --graphiql "/" \
  --enhance-graphiql \
  --allow-explain \
  --enable-query-batching \
  --legacy-relations omit \
  --connection "$DATABASE_NAME" \
  --schema "$SCHEMA_NAME"
```

### Expected results

The endpoint `searchPowerups` should have both an `orderBy` and a `condition` field.

### Actual results

Both `orderBy` and `condition` are missing.

### Additional context

All provided above

### Possible Solution

Unfortunately, no idea
