Missing schema for block config

Problem/Motivation

When running functional tests based on BrowserTestBase, I am seeing this error:
Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for block.block.THEME_datemodifiedblock with the following errors: block.block.THEME_datemodifiedblock:settings.date_modified missing schema

where THEME is the machine name of the theme.

Steps to reproduce

Run functional tests on a site with date_modified_block in use. (It seems likely that other blocks would trigger this as well.)

Proposed resolution

I was able to fix it by creating a complete config schema for block.block.THEME_datemodifiedblock. I don't think that should be necessary. I think this should be solved in wxt_library by defining a schema for settings.date_modified in block.settings.date_modified_block or something like that. Then it should be inherited by block.block.THEME_datemodifiedblock.

(This can also be worked-around by setting $strictConfigSchema = FALSE.)

Remaining tasks

Figure out what to do. Implement.

User interface changes

None.

API changes

None.

Data model changes

None.