Thank you for your quick response

My project settings is here

In the codec setting I have not customize anything

I tried using this setting to render a standard footage from my sony ax-100 camcorder and everything works fine

That's exactly the same I did.
Very strange that render with you.
Besides to reset Vegas to the default values is should not know anything to help you further.
To reset those Vegas default settings first be sure you saved your project, than start Vegas with hold Ctrl and Shift buttons and check this box that will pop-up.

When Vegas started again try your project again.
 

Try rendering without using Intel (QSV). Render only with the CPU.

I do not have the QSV rendering enabled (It does not matter because my processor does not have QSV for the H265)

I reset Vegas to the default value but nothing has changed

I tried to render one image of duration 5 and then 10 seconds - the result was fine.

When I tried the duratin of 20s, the result was 11:06 and the frame rate was 44,698 Hz, which approximately corresponds to 20 * 25 / 44,698 = 11,19 (the number of times the frame rate increases, the duration is reduced)

I do not have the QSV rendering enabled (It does not matter because my processor does not have QSV for the H265)

You showed us here the rendersettings you used of the Magix HEVC Intel QSV render option.
If you should have no processor to do that ( in fact you have one with your Intel® HD Graphics 4000) that option and codec would not show up in the list.
The only thing you have to do is to switch to the Mainconcept HEVC in the customized setting by choosing it in the options you have.
Those are (your screenshot)

and after making the choice

to choose the mainconcept your on board graphics GPU) will not be used, only the CPU.

I do not have the QSV rendering enabled (It does not matter because my processor does not have QSV for the H265)

You showed us here the rendersettings you used of the Magix HEVC Intel QSV render option.
If you should have no processor to do that ( in fact you have one with your Intel® HD Graphics 4000) that option and codec would not show up in the list.
 

That is a bug with vegas16 then. Magic HEVC gives default option of Intel QSV even though his Quicksync does not do h.265 encoding & should not show as option.

I get the same issue here in VP16 build 352.

My Intel i7-5960X does not have QSV but when I choose MAGIX HEVC/AAC MP4 I only see templates with "(Intel QSV)" after them.

If I render stills with "Intel QSV" selected as the Encode mode, the rendered video stream is too short.

If I render stills with "MainConcept HEVC" selected, the rendered video is OK.

If I render video media (not stills) with either "Intel QSV" or "MainConcept HEVC" selected, the result is OK but it is subtly different.

An "Mainconcept HEVC" render requires additional activation whereas the "Intel QSV" does not.

I'm assuming "Intel QSV" encode mode falls back to a CPU (non-QSV) encode if QSV is not available, but using a different encoder than the Mainconcept one. There is evidence of that in this old comment (in relation to the original "Intel HEVC" encoder) from a Magix developer (@VEGASHeman Could you possibly clarify?).

If that's true then I see 2 issues here:

1. The term "Intel QSV" in the template name and in the "Encode mode" menu is misleading if you haven't got HEVC-capable QSV and would be better if it was just "Intel".
2. Intel CPU HEVC rendering of stills (without QSV) results in an incorrectly short video stream.

If I render stills with the "Intel HEVC" encoder, the rendered video is OK.

@NickHope
👍
I was also looking for another "proof" than rendering and found when I disable my Intell GPU or both Intell and Nvidia that renderoption will still be shown in Magix HEVC, while in the Magix AVC renderoptions they appear as they are enabled or disabled.
Here a screenshot of both disabled GPU's and the Magix AVC renderoptions

and here of the Magix HEVC options