MySQL not using index WHERE IN - mysql

Question

I have a simple SELECT query that's using WHERE IN across 5 integer values to get 50 results in a table of roughly 20 million rows. Here's the query:
select `prf_profiles_id` as `id`, `first`, `last`, `suffix` from `prf_names`
where prf_names`.`src_sources_id` in (9, 10, 11, 34, 37) limit 50 offset 0;
And the explain values for the query:
select_type: SIMPLE
table: prf_names
type: ALL
possible_keys: prf_names_src_sources_id_index
key: NULL
ref: NULL
rows: 20012960
Extra: Using where
As can be seen, the query knows the index is possible, but chooses not to use it and as a result the query takes about 4.5 seconds.
src_sources_id is indexed, and contains about 20 distinct values applied to all 20 million rows.
I know that the index would help immensely because when I run the same query using FORCE INDEX:
select `prf_profiles_id` as `id`, `first`, `last`, `suffix` from `prf_names`
force index (prf_names_src_sources_id_index)
where `prf_names`.`src_sources_id` in (9, 10, 11, 34, 37) limit 50 offset 0;
The query takes under 0.0 seconds.
I'd like to avoid forcing MySQL to use the index as the query is being called through an ORM and forcing it would defeat some of the purpose of having the ORM in the first place.
What can I do to ensure MySQL uses this index going forward?
EDIT:
Here's the create table statement:
CREATE TABLE `prf_names` (
`prf_profiles_id` int(10) unsigned NOT NULL,
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`created` datetime NOT NULL,
`verified` datetime NOT NULL,
`title` varchar(20) COLLATE utf8_unicode_ci DEFAULT NULL,
`first` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
`middle` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
`last` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
`suffix` varchar(10) COLLATE utf8_unicode_ci DEFAULT NULL,
`sanitized` enum('yes','no') COLLATE utf8_unicode_ci NOT NULL DEFAULT 'no',
`src_sources_id` int(10) unsigned NOT NULL,
PRIMARY KEY (`prf_profiles_id`,`id`),
UNIQUE KEY `prf_names_id_unique` (`id`),
KEY `prf_names_first_index` (`first`),
KEY `prf_names_middle_index` (`middle`),
KEY `prf_names_last_index` (`last`),
KEY `prf_names_src_sources_id_index` (`src_sources_id`)
) ENGINE=InnoDB AUTO_INCREMENT=31633081 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci