I added an additional utf-8 character encoding meta tag. The thing is that KF8 is produced by generating an EPUB3 fixed layout file that is then transformed into a KF8.
Actually that warning comes from the EPUB3 validator, which is complaining about the double charset declaration:

<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<meta charset="utf-8"/>

The thing is that:

1. If we remove the first meta tag, things break up in Kindle Previewer for Mac
2. If we remove the second meta tag, things break up in the online Kindle Previewer (actually this is how we always exported KF8, didn't know about online Kindle Previewer issue)
3. If we keep it both, EPUB3 validator complains

May guess is that this is actually an issue by the online Kindle Previewer.

- Do you see the content correctly on a physical device? If yes, do you see it correctly also with the production pubcoder release or only on the beta?
Can you try to publish the file to Amazon Store just to be sure that their validator doesn't complain too?

If it doesn't, the solution may be to simply muting the EPUB3 validator complain. Let me know how the publishing tests go.

Hello Daniel,
I tried to add an additional UTF-8 encoding declaration for KF8 export in a beta release, please try it and let me know if this fixes the issue:
https://www.dropbox.com/s/tnv1pex9f0v3091/PubCoder-bin-20230307-4.3.4.1191.zip?dl=0

Hello Gabriel,
sadly, those are problems with the ebook viewers that are not fully compatible with EPUB3, especially when it comes to advanced features like soundtrack, read aloud, interactivity. Those problems and incompatibilities is one of the reasons why we added other export options, like XPUB/PubReader, HTML, Native Apps, Shelf

Hello Ramon,
Display options depend on the reader app. Most EPUB readers will display double pages in landscape automatically, when using portrait (vertical) page formats, but other don't. Same for the page flipping effect

What OS are you using? Windows or Mac? Just quickly tried on my Mac and can't seem to reproduce the issue.

I think you could use the Soundtrack for this.

Hi Milena, the latest version that supported Android 4.4 at the time should be PubCoder 3.8 (released early 2020), but that does not mean that it will work now with current versions of macOS or Windows or tooling required to compile an Android app.

Hello everyone, we just released PubCoder 4.3.3 which fixes the problem that prevented PubCoder (4.3.2) from completing startup on macOS 10.5.x Catalina and macOS 11.x Big Sur.
You download it from our download page at https://pubcoder.com/download

Sorry for the inconvenience. Please let us know if everything works as expected now.

hello,
what version of macOS are you using?
Do you at least see the pubcoder menubar? Is PubCoder responding or the app is not responding?
Can you send a screenshot with pubcoder running in front with the mouse cursor over the menubar?

Some days ago we updated some certificates on our servers, so maybe it's just a temporary connection issue? For the vast majority of our users the latest version seem to work without any problems, but still we're investigating the issue.

In the meanwhile, you can download the older version of pubcoder here: https://docs.pubcoder.com/pubcoder_release_notes.html#version-431---november-10-2022

here it is again https://www.dropbox.com/s/yeka55m06o59se2/clone.pubcoder?dl=0