Preview-Button fills field with serialized array as value

In Version 2.4.3 I found a new problem with showing serialized values instead of an empty field.

I have an custom field template with three optional text-fields. As i leave them empty and click the “Preview Changes” Button, the first custom field gets a value like @(a:1:{i:0;a:1:{i:0;s:0:””;}})@. It also happens with content: value “KAB” gets @(a:1:{i:0;a:1:{i:0;s:3:”KAB”;}})@ These Extra-Characters are not visible in the Backend, the mess is only visible in Frontend! It is NOT visible in the preview of the page BUT is visible in the current published version of the page. Strangely it only happens with the first field, not the other ones.

Workaround: when I save / published the post again and stay away from the preview-Button, everything is alright. This problem did not occur in version 2.4.1.
It seems to have something to do with serializing the value and saving this as a real value.

Any help would be appreciated.
Thanks!

Note: i still have problems with missing values in previews of draft-Posts as described here – the above described bug occurs with function get_post_meta() or without it.
https://www.ads-software.com/support/topic/preview-doesnt-show-current-data/