Steps to reproduce:
 Open a new text document.
 Disable option “LibreOffice Writer → Formatting Aids → Cursor in protected areas“.
 Insert a heading.
 Insert a protected (against manual changes) Table of Contents above the heading line: Insert > Indexes and Tables > Indexes and Tables... > Tab "Index/Table", Type: "Table of Contents" > OK.
 Now it is not possible to insert a paragraph above the table of contents. Expected: It should be possible to move the cursor in front of the “T“ of “Table of Contents“. Then Alt+Enter should insert a new paragraph above the table.
If you enable the option in step  it is possible to move the cursor in front of the “T“, but it's still not possible to insert a new paragraph.
See also enhance proposal in bug 51689 according creating paragraphs above and below of tables, indexes and sections.
Operating System: Windows 7
Version: 188.8.131.52 release
Reproducible but, for me, it is not a bug, only an enhancement.
That said there is an easy workaround: remove the ToC, insert the paragraph, insert the ToC. As inserting a ToC is automatic, it is very easy to do.
Note: IMHO, the need of this enhancement comes from a workflow that does use templates as it should. It is weird to create a ToC at the top of the document, the beginning of the document is the place of the title, not of the ToC.
Best regards. JBF
Enter at the very first position in the table adds the break before it now. Tested with
Build ID: 6.2.8-3
CPU threads: 8; OS: Linux 5.3; UI render: default; VCL: kde5;
Locale: de-DE (en_US.UTF-8); UI-Language: en-US