doxygen version: 1.8.15
Operating system: Ubuntu 18.04 LTS
Doxyfile: default with the exceptions described below

Generated navtree root node has changed since version 1.8.13 in case the PROJECT_NAME is empty and a markdown document is used as main page.

Suppose the configuration is

USE_MDFILE_AS_MAINPAGE = src/mainpage.md
PROJECT_NAME =

and there is the following src/mainpage.md:

Main Page {#mainpage}
=========

Section 1 {#section1}
=========
Test text for Section 1 

Section 2 {#section2}
=========
Test text for Section 2

The navtree generated with 1.8.13 was:

Main Page
    Section 1
    Seciont 2

The navtree generated with 1.8.15 is:

My Project
    Main Page
        Section 1
        Seciont 2

So it is not possible anymore to have the title of the main page as root node and the section of the main page as top level entries. Is it a bug or the intended behaviour?