Changes to a block's text field not saved when ckeditor source mode is active

Problem/Motivation

On a layout builder page I am seeing data loss when saving an existing block with source mode enabled in ckeditor. At least the data loss is quite obvious ;)

Steps to reproduce:

• Add block to section
• Create custom block
• Edit text in body field
• Save block
• Edit block
• Switch body field to ckeditor source mode
• Edit text
• Save
• Changes are NOT saved!

Once I changed back to non-source mode before saving the changes are saved properly.

Variation

This is a bit more severe with ckeditor_config (on a core 8.7.8 install with a lot modules installed) module enabled.

Steps to reproduce:

• Add startupMode = source to ckeditor_config settings in basic_html
• Edit block
• Switch body field to source mode
• Edit text
• Switch to non-source mode (this makes it work in the first example)
• Save
• Changes are NOT saved!

(Disclaimer: In the variation I am not 100% sure if it is because of the mentioned ckeditor_config settings, at least I haven't verified on a vanilla drupal install)

Proposed resolution