Sorting Student Report by Completed Column Fails

### 1. Reproduction Steps

+ Log into the LifterLMS Academy site
+ Go to Reporting and Click on Courses
+ Choose a Course
+ Select the Students Tab
+ Try and sore by the "Completed Column"
+ The sort fails as seen here: https://www.evernote.com/l/AnjJNtm2siBJtrkvad5kKbiO76cVJ-T_DlU

### 2. Expected Behavior

+ The sort would return results (but didn't)

### 3. Actual Behavior

+ IThe sort fails as seen here: https://www.evernote.com/l/AnjJNtm2siBJtrkvad5kKbiO76cVJ-T_DlU (but isn't supposed to)

### 4. Error Messages / Logs

+ "No results were found."

### 5. System Report

Many issues are the result of theme or plugin conflicts. Including information about your WordPress version, your plugins (and their versions), and your theme (and its version) will help us resolve conflicts.
We've created the [LifterLMS System Report](https://lifterlms.com/docs/how-to-use-the-lifterlms-system-report/) to help with this

Wordpress
-------------------------------------------

Home Url: https://academy.lifterlms.com
Site Url: https://academy.lifterlms.com
Login Url: https://academy.lifterlms.com/wp-login.php
Version: 5.0.3
Debug Mode: No
Debug Log: No
Debug Display: Yes
Locale: en_US
Multisite: No
Page For Posts: Blog (#21) [https://academy.lifterlms.com/blog/]
Page On Front: Welcome 2 (#6145) [https://academy.lifterlms.com/]
Permalink Structure: /%postname%/
Show On Front: page
Wp Cron: No

Settings
-------------------------------------------

Version: 3.28.2
Db Version: 3.28.2
Course Catalog: Courses (#9) [https://academy.lifterlms.com/courses/]
Membership Catalog: Memberships (#8) [https://academy.lifterlms.com/memberships/]
Student Dashboard: My Courses (#11) [https://academy.lifterlms.com/my-courses/]
Checkout Page: Enrollment (#10) [https://academy.lifterlms.com/purchase/]
Course Catalog Per Page: 10
Course Catalog Sorting: menu_order,ASC
Membership Catalog Per Page: 10
Membership Catalog Sorting: menu_order
Site Membership: Not Set
Courses Endpoint: my-courses
Edit Endpoint: edit-account
Lost Password Endpoint: lost-password
Vouchers Endpoint: redeem-voucher
Autogenerate Username: yes
Password Strength Meter: no
Minimum Password Strength: weak
Terms Required: yes
Terms Page: About (#6097) [https://academy.lifterlms.com/about/]
Checkout Names: required
Checkout Address: hidden
Checkout Phone: hidden
Checkout Email Confirmation: no
Open Registration: yes
Registration Names: hidden
Registration Address: hidden
Registration Phone: hidden
Registration Voucher: optional
Registration Email Confirmation: no
Account Names: optional
Account Address: hidden
Account Phone: hidden
Account Email Confirmation: no
Confirmation Endpoint: confirm-payment
Force Ssl Checkout: no
Country: US
Currency: USD
Currency Position: left
Thousand Separator: ,
Decimal Separator: .
Decimals: 2
Trim Zero Decimals: no
Recurring Payments: no
Email From Address: team@lifterlms.com
Email From Name: LifterLMS Academy
Email Footer Text: Got questions? Hit reply. We're here for you.
Email Header Image: 22630
Cert Bg Width: 800
Cert Bg Height: 616
Cert Legacy Compat: yes

Gateways
-------------------------------------------

Stripe: Enabled
Stripe Test Mode: Disabled
Stripe Logging: yes
Stripe Order: 1
Manual: Disabled
Manual Logging: no
Manual Order: 1

Server
-------------------------------------------

Mysql Version: 5.6.10
Php Curl: Yes
Php Default Timezone: UTC
Php Fsockopen: Yes
Php Max Input Vars: 10000
Php Max Upload Size: 100 MB
Php Memory Limit: 512M
Php Post Max Size: 100M
Php Soap: Yes
Php Suhosin: No
Php Time Limt: 60
Php Version: 7.2.12-1+ubuntu16.04.1+deb.sury.org+1
Software: Apache
Wp Memory Limit: 512M

Browser
-------------------------------------------

HTTP USER AGENT: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36

Theme
-------------------------------------------

Name: LifterLMS LaunchPad
Version: 2.4.6
Themeuri: https://lifterlms.com/launchpad/
Authoruri: https://gocodebox.com
Template:
Child Theme: No
Llms Support: No

Plugins
-------------------------------------------

Akismet Anti-Spam: 4.1.1
Beaver Builder Plugin (Pro Version): 2.2.1
Drift: 1.8.4
LifterLMS: 3.28.2
LifterLMS Assignments: 1.0.0
LifterLMS Helper: 3.0.2
LifterLMS Labs: 1.5.2
LifterLMS Modifications: 0.1
LifterLMS Stripe Payment Gateway: 4.3.4
OptinMonster API: 1.6.2
Redirection: 3.7.3
Ultimate Addons for Beaver Builder: 1.15.0
User Switching: 1.4.2
WP Fusion: 3.22.3

Integrations
-------------------------------------------

BbPress: No
BuddyPress: No

Template Overrides
-------------------------------------------

### 6. Browser, Device, and Operating System

+ If the bug occurs in a specific browser, it is very useful in helping us track down the specifics using this information (as well as version numbers).