Possible issue when cloning a course with quotes in the classic block

I'm not able to reproduce this issue on my end, but this has been reported by one of our customers, and I want to keep track of it in case other similar cases will pop up:
HS-108804

The customer says:

> I noticed that when I clone a course there is extra code added to the lessons. Wherever there is a " in the html, it adds \"
>
> For example:
>
>

>
>
• Story Maps 101

>
>
• Blog Post: 10 Essential Steps for Story Map Success

>
>

>
> This breaks all of the links, embeds, inline styling, etc. The only components I have in the affected courses are Classic blocks. I haven't tested this with other block types.

### System Information

System Report

```
System Report:
Wordpress
-------------------------------------------

Home Url: https://pxxx.org
Site Url: https://pxxx.org
Login Url: https://pxxx.org
Version: 5.3.2
Debug Mode: No
Debug Log: No
Debug Display: Yes
Locale: en_US
Multisite: No
Page For Posts: Not Set
Page On Front: Home (#17) [https://pxxx.org]
Permalink Structure: /%year%/%monthnum%/%day%/%postname%/
Show On Front: page
Wp Cron: Yes

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

Version: 3.37.6
Db Version: 3.37.6
Course Catalog: Programs (#6) [https://pxxx.org/programs/]
Membership Catalog: Memberships (#7) [https://pxxx.org/memberships/]
Student Dashboard: My Account (#9) [https://pxxx.org/my-account/]
Checkout Page: Purchase (#8) [https://pxxx.org/purchase/]
Course Catalog Per Page: -1
Course Catalog Sorting: menu_order,ASC
Membership Catalog Per Page: 9
Membership Catalog Sorting: menu_order
Site Membership: Not Set
Courses Endpoint: my-programs
Edit Endpoint: edit-account
Lost Password Endpoint: lost-password
Vouchers Endpoint: vouchers
Autogenerate Username: yes
Password Strength Meter: yes
Minimum Password Strength: strong
Terms Required: no
Terms Page: Not Set
Checkout Names: required
Checkout Address: required
Checkout Phone: required
Checkout Email Confirmation: yes
Open Registration: yes
Registration Names: required
Registration Address: hidden
Registration Phone: hidden
Registration Voucher: optional
Registration Email Confirmation: yes
Account Names: required
Account Address: hidden
Account Phone: hidden
Account Email Confirmation: yes
Confirmation Endpoint: confirm-payment
Force Ssl Checkout: no
Country: US
Currency: USD
Currency Position: left
Thousand Separator: ,
Decimal Separator: .
Decimals: 2
Trim Zero Decimals: yes
Recurring Payments: yes
Email From Address: --
Email From Name: --
Email Footer Text:
Email Header Image:
Cert Bg Width: 800
Cert Bg Height: 616
Cert Legacy Compat: no

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

Manual: Disabled
Manual Logging: no
Manual Order: 1

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

Mysql Version: 5.7.27
Php Curl: Yes
Php Default Timezone: UTC
Php Fsockopen: Yes
Php Max Input Vars: 1000
Php Max Upload Size: 50 MB
Php Memory Limit: 512M
Php Post Max Size: 100M
Php Soap: Yes
Php Suhosin: No
Php Time Limt: 3600
Php Version: 7.3.12-1+ubuntu18.04.1+deb.sury.org+1
Software: Apache
Wp Memory Limit: 40M

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

HTTP USER AGENT: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.79 Safari/537.36

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

Name: Astra (child)
Version: 1.0.0
Themeuri:
Authoruri: http://wpastra.com/about/
Template: astra
Child Theme: Yes
Llms Support: Yes

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

3D FlipBook - Light Edition: 1.9.20
Advanced Custom Fields PRO: 5.8.7
bbPress: 2.6.3
BuddyPress: 5.1.1
Gravity Forms: 2.4.16
LifterLMS: 3.37.6
LifterLMS Advanced Quizzes: 1.0.10
LifterLMS Assignments: 1.1.4
LifterLMS Customizations: 1.0.0
LifterLMS Helper: 3.0.2
LifterLMS Labs: 1.5.2
LifterLMS MailChimp Add-on: 3.1.2
LifterLMS WooCommerce: 2.0.12
Loco Translate: 2.3.1
MathJax-LaTeX: 1.3.11
Popup Maker: 1.8.14
Responsive Lightbox: 1.3.2
WooCommerce: 3.8.1
WooCommerce Customer/Order/Coupon Export: 5.0.6
WooCommerce Stripe Gateway: 4.3.1

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

BbPress: Yes
BuddyPress: Yes
WooCommerce: Yes
LifterLMS MailChimp: Yes

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

content-certificate.php (ver: 3.18.0): /nas/content/live/pxxx/wp-content/plugins/lifterlms-customizations-master//templates/ (ver: 3.18.0)
content-single-lesson-after.php (ver: 3.0.0): /nas/content/live/pxxx/wp-content/plugins/lifterlms-customizations-master//templates/ (ver: 3.0.0)

```

I wonder if there's any connection with #1026
except for our plugins, anything else shared? ACF Pro at least, as far as I can see.

Adding the "bug" label, just because it has been reported as such.