As this is a functional description of user experience, I would consider the lack of "applied style" to be a deletion.
In terms of it being a display issue, that would imply that the correct style info is present but the rendering system (i.e., browser) is not interpreting properly.
That is not the case. Applying new formatting (cell borders, bg color, etc) to new tables in the WYSIWYG does not result in production of correct HTML.
Existing table cells from previous Alfresco version wiki that had such formatting, have class tags with values such as "xl119". Assumedly, these were defined in a stylesheet that did not migrate with the upgrade or that is no longer being properly referenced. In either case, the assertion that the "style is still there in the editor" is wrong. Any such cell formatting from previous version or newly applied is not reflected in the editor.