Time field is filled with 0:00

After the migration from 10.4 to 11.5 is the time field always filled with 0:00 if i create a new record.

Typo3 displays the message: ‘Column ‘time’ cannot be null’ if i try to store the record without changes.

Here is my tx_participants_domain_model_event.php

'time' => [
           'label' => 'LLL:EXT:participants/Resources/Private/Language/locallang_db.xlf:tx_participants_domain_model_event.time',
           'config' => [
               'type' => 'input',
               'renderType' => 'inputDateTime',
               'dbType' => 'time',
               'eval' => 'time',
               'default' => 68400, // 19:00h
               'mode' => 'useOrOverridePlaceholder'

and here is the important line from my tables.sql:

   time VARCHAR(10) DEFAULT '00:00:00' NOT NULL,	

Can anyone else relate to this as well? Is this a bug?

Unfortunately I can’t detect a bug, although I have a login at https://my.typo3.org/

Thanks for your help…