Pulling a random value out of a table is returning a null value

I have a stored procedure that I've used to 'de-identify' client information when I want to use it in a test environment. I am replacing actual names and addresses with random values. I have database tables in a database called dict (for dictionary) for female names, male names, last names, and addresses.

Each of these has a field called f_row_id that is a sequential number from 1 to x, one for each record in the table.

We recently upgraded to mySQL 8 and the stored procedure quit working. I ended up with NULL for every field where I tried filling in a random value out of the other table. In trying to find what will now work, I'm unable to get the following query to work as I expect:

SELECT
    f_enroll_id,
    (SELECT f_name FROM dict.dummy_female_first_name fn WHERE fn.f_row_id = (FLOOR(RAND() * 850)  + 1) LIMIT 1)
FROM
    t_enroll

My data table (that I eventually want to have contain random names) is called t_enroll. There is an ID field in that (f_enroll_id) I want to get a list of each ID and a random first name for each record in that table.

There are 850 records in the table of random first names (dummy_female_first_name) (in my stored procedure this is a session variable that I compute at the start of the procedure).

When I first tried running this I got an error that my sub-query returned more than one value. I don't understand why it would do that since (FLOOR(RAND() * 850) + 1) should return a single integer. So I added the LIMIT 1. But when I run this, about half of the returned rows have NULL for the first name.

I have verified that all the rows in my first name table have a row ID, that the row ID is unique, and there not any gaps in the numbers.

What do you think is causing this?

Thanks in advance!