This problem arose suddenly. Whenever I try to write Arial in any Microsoft Office application (including WordPad) the text appears italic. Changing it to italic and back just does nothing, the text remains italic in both settings.
This is not a system wide issue with the font itself, for example webpages for which I set the font to Arial in 'Inspect Element' show up as expected. If I upload the same Powerpoint file to OneDrive and view it in the WebApp, it is also displayed normally. Printing the local file as PDF (I don't have a physical printer), however, also exports it italicised, even if viewed in different applications (Edge, Adobe Acrobat) or uploaded to OneDrive.
Searching the internet, I found several posts about a similar issue where Arial Black always showed up as italic, but I never found a solution to that either.
(Apart from the Office365 suite and WordPress, I don't think I have any other text editing programs installed that let me choose the font.)
Update: After having solved it, I believe that it also didn't render the webpage with Arial, but used some fallback that looked very similar (or maybe pulled it from somewhere online?). Nevertheless, it appears like it actually was a system-wide error and the Word WebApp used a remote version of Arial to render it correctly.