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It works because web browsers have all video codecs they need build-in. You should raise this issue with Mint and let them know you can't play mp4 videos outside the gstreamer platform

I agree. I have been pushing for this for a few years now. So far, no luck. Good thing someone else is also asking for that.

I do not use mint, nor do I use Presentations regularly, and those times I have I haven't linked any videos.

Take a look at https://wiki.linuxmintusers.de/index.php?title=Mint-meta-codecs . They list all media codecs available for Mint, this may help.

Is possible you need libmpeg and/or ffmpeg ...

AS the error says... You are missing a codec. Ensure you have all the gstreamer codecs (good, bad, ugly), as well as other codecs that are not part of of the gstreamer architecture: libx264, ffmpeg and so on. Mos distributions have an article explaining how to install all media codecs.
You may then ...

Yes the validator identifies the provided file as ODT 1.0, but if you look closely:

Lorem.odt/mimetype: Error: There shall be no extra field for the 'mimetype' zip entry of ODF package 'Lorem.odt'!
Lorem.odt/mimetype: Error: For more information on ODF package conformance see https://docs.oasis ...

Out of curiosity I used https://odfvalidator.org to validate if a very basic odt document (just, text. No styling, no comments, no changes tracked) created in TextMaker was valid or not. It is not. The document is NOT conformant ODF1.0.

Here are the results for the attached dummy document:

Lorem ...

Native support for open source files. Textmaker already opens and saves odt - cheers!; now, planmaker and presentations should open and save odp and ods. This is still an obstacle for those who want to transition with open source format - specially with the recent concerns regarding MS and North ...

Hello, I‘d like to add my request for a flatpak version, since more and more GNU/Linux Distros are immutable/atomic. Fedora, (open)Suse, Ubuntu (upcoming) just to name the big ones. Almost all Gaming-Distros are somehow immutable (currently there is a massive movement from Windows to GNU/Linux ...

This issue has not been fixed.

You can. Just duplicate the slide colour scheme, edit, and change according to your needs, then apply to the desired slides.

To duplicate the colour scheme, simply select your current scheme and click on "New scheme"

Try Ctrl+Enter

Indeed, the issue is that there are no option for smart quotes in Spanish and the auto option does just what I reported. I suppose then that I should request that smart quotes support is added to TextMaker.

Edit: For not I have set smart quotes to work as in English, while manually using « ». This ...

Check out the Web font readme (webfont_readme.pdf), all you need is there

When language is set to Spanish, quoting marks are set incorrectly when they are nested. For example:

«Antonio me dijo: «Vaya «cacharro» que tiene Julián»».

That's wrong. The correct way would be:

«Antonio me dijo: “Vaya ‘cacharro’ que tiene Julián”»

Meaning that when you nest the order is ...

When saving to ODT, all formatting and styles applied to comments, including the default «comment text» is ignored and «Normal» is used instead.

Step to reproduce:

1. Add a comment to text
2. Add formatting or any style
3. Safe as ODT.
4. Close the file
5. Load the ODT file