Your problem made me curious, so I played a bit. I took one of my slide presentations and applied the Metropolis master slide theme to all the slides. I then exported the file as PDF, HTML, and XHTML and then sent them to my Samsung A51 smartphone.
The PDF displayed just fine on my Samsung phone. I then opened the file on my iPad and immediately saw what you are talking about. The text was lost on all my slide titles and on some slides, it was cut off. It only happened on the Apple device, and not on the Android device.
I then used my original LO file, which was NOT made with the Metropolis master slide theme. I used a master slide that I had created myself and it loaded just fine on my iPad.
It appears as if the problem is within the Metropolis master slide theme.
As some suggested using HTML, I tried exporting the file as both HTML and XHTML, and both were much more challenging and I received a wide variety of results depending on the various HTML variables I applied. However, despite all my experimentation, I didn't find any HTML export solution that gave me everything. Some resulted in a separate file for each slide, some resulted in a loss of slide structure and others resulted in a loss of graphics. In short, I wasn't satisfied with any of the HTML solutions.
If I were you, I would try a different master slide theme. The Metropolis clearly creates a problem, but I don't know why. It's not just a font issue. I changed the font on my Metropolis based presentation to Sitka Text (the font used in my self-created theme) and it was still getting cut off on the Apple device.
Virgil