I am reporting this issue on behalf of this forum post, since the user who reported it there probably doesn't have a github account. I will quote their description below, and then add in my confirmation

Description of the Issue

Hi,

I’ve noticed that when I start a new instance of Notepad++ the “New 1” document isn’t set as HTML (my New document | Default language setting).

Works fine if I create a new document via File | New.

Seems like it’s started happening since v8.4.7 - just wondering if this is happening for others too?

thanks, Craig.

My Debug Info:

Notepad++ v8.4.7 (64-bit)
Build time : Nov 1 2022 - 23:31:11
Path : C:\Program Files\Notepad++\notepad++.exe
Command Line :
Admin mode : OFF
Local Conf mode : OFF
Cloud Config : OFF
OS Name : Windows 10 Enterprise (64-bit)
OS Version : 21H2
OS Build : 19044.2251
Current ANSI codepage : 1252
Plugins :
BetterMultiSelection (1.5)
DSpellCheck (1.4.24)
HexEditor (0.9.12)
HTMLTag (1.3.5)
mimeTools (2.9)
NppExport (0.4)
PythonScript (2)
QuickColorPicker (2.1.2.1)
XBrackets (1.3.1)

[end of Craig's text]

Steps to Reproduce the Issue

I used these steps to confirm Craig's results.

0. Start with fresh 8.4.7 portable, so no other settings or plugins get in the way
1. Settings > Preferences > New Document > Default Language = HTML
2. Close all open files (to make sure the session is empty)
3. Exit Notepad++
4. Run notepad++ again: the default new 1 is "Plain Text", not "HTML"
5. Use File > New or equivalent, and new 2 is correctly "HTML"
6. Close new 1 and leave new 2 open
7. Use File > New or equivalent, and the newly-created new 1 is is also correctly "HTML".
8. Close new 1.
9. Exit Notepad++
10. Run Notepad++ again: the default new 1 is "Plain Text", not "HTML"

Expected Behavior

When Notepad++ is opened with no files in the current session, I expect it to follow the Settings > Preferences > New Document > Default Language for new 1.

In v8.4.6, the new 1 for a new run was correctly HTML (ie, followed the expected behavior).

Notepad++ v8.4.6   (64-bit)
Build time : Sep 25 2022 - 19:51:39
Path : C:\usr\local\apps\npp\npp.8.4.6.portable.x64\notepad++.exe
Command Line : 
Admin mode : OFF
Local Conf mode : ON
Cloud Config : OFF
OS Name : Windows 10 Enterprise (64-bit) 
OS Version : 20H2
OS Build : 19042.2251
Current ANSI codepage : 1252
Plugins : 
    mimeTools (2.8)
    NppConverter (4.4)
    NppExport (0.4)

Actual Behavior

When Notepad++ is opened with no files in the current session, new 1 was "Normal text file" despite Settings > Preferences > New Document > Default Language being set to HTML. I confirmed this regression in my portable v8.4.7.

Notepad++ v8.4.7   (64-bit)
Build time : Nov  1 2022 - 23:31:11
Path : C:\usr\local\apps\npp\npp.8.4.7.portable.x64\notepad++.exe
Command Line : 
Admin mode : OFF
Local Conf mode : ON
Cloud Config : OFF
OS Name : Windows 10 Enterprise (64-bit) 
OS Version : 20H2
OS Build : 19042.2251
Current ANSI codepage : 1252
Plugins : 
    mimeTools (2.9)
    NppConverter (4.5)
    NppExport (0.4)

Since this worked as expected in v8.4.6 and now doesn't in v8.4.7, I assume this regression came about because of #11504 (but that is just an assumption)

--
update: I removed my use of "instance" (though I kept it in the quote from Craig): the problem is not specific to multi-instance mode. As the steps I described show, it happens even when you just have one instance of Notepad++ running at a time, with the unedited (default setting) of single-instance-mode.