When I'm adding my custom block and then trying to edit it in "Edit as HTML" mode, any changes reset to the previous state every time when I click outside the editing window. Let say default HTML in my block is <div class="wp-block-my-custom-video-box"></div>, if I make some changes, let say <div class="wp-block-my-custom-video-box">some text</div> some text will disappear. This issue related to WordPress ver. 5.3.2, with ver. 5.2.5 everything works fine. Help me figure out, please, what can cause such strange behavior?

this is a simplified example of the component: