Mysql – Large database table queries return empty set

I'm trying to understand a weird problem with a MySQL database on one of my websites.

When I loaded a big set of data from a CSV file (2,200,000 records with a single column, using LOAD DATA) into a table, my application started to misbehave.

After some time looking at the malfunctioning, I found out that some queries on that big table aren't working as expected.

I've tried the simplest query possibile to look for a certain entry in that table: querying for a certain value (that I know it's present) for a varchar(45) primary key column (ex. SELECT column_name FROM table_name WHERE column_name = "string_value"). Surprisingly the server returned me an empty set.
I also tried SELECT COUNT(*) FROM table_name WHERE column_name = "string_value" and it, returned zero.

When I do a similar look-up for a value in a similar table (but with just a couple entries), the query works correctly and returns me a single row.

My question: is it possible that this case is some kind of bug with the MySQL database server?

I'm using MySQL version 5.6.35-80.0-log.

The two example tables structure contain two columns: a varchar(45) primary key and a bit(1) "boolean".
The first column contains code string consisting of alphanumeric characters