3.9.9: Partial/ Include help needed

When I try to include the header as a partial include in macOS Big Sur 11.4 and Wappler 3.9.9, the result looks okay in the design view, but when I try to view the result in Firefox 89.0.2 (macOS 64-bit), I get the text of the include, but not the header (see images below). Also, this doesn’t work if I call the include/partial file Header.html - which I should have done (fails either way).

Partial include does not work1670×968 139 KB

Partial include does not work21592×768 151 KB