[Widget Screen] Max of 9 custom sidebars shown

[annezazu]

Describe the bug

If a site has more than 9 custom sidebars, only the first 9 will show in the new widget screen. The rest are automatically hidden.

To reproduce
Steps to reproduce the behavior:

1. Deactivate Gutenberg temporarily and install a plugin like this to create over 9 custom sidebars. If you already have over 9 sidebars, you can skip this step!
2. Enable and activate Gutenberg.
3. Head to Appearance > Widgets.
4. See only 9 custom widget areas.

Expected behavior

I expected to see all 13. This was also reported in the forums here: https://wordpress.org/support/topic/gutenberg-widgets-block-editor-is-a-mess-of-errors/

Screenshots

Editor version (please complete the following information):

• WordPress version: 5.5.1
• Does the website has Gutenberg plugin installed, or is it using the block editor that comes by default? Gutenberg plugin.
• If the Gutenberg plugin is installed, which version is it? 9.1.1

Desktop (please complete the following information):

• OS: MacOS
• Browser: Chrome
• Version: 85.0.4183.121