Special characters printed bold on iOS

Question

On some parts of the site I am maintaining German umlauts are displayed in bold (see screenshot). I am using font-family: Source Sans Pro, Arial, sans-serif; with font-weight: 300. The font is embedded from Google Fonts.

Changing the font weight to 400 and above or using a different font works flawlessly. The thing that really weirds me out that it only occurs for the lower paragraph. I tried disabling every font-related style in Safari Web Inspector for both paragraphs without success.

This seems to only happen in Safari and Chrome on iOS (Chrome on macOS and Android works fine).

Affected site: https://sentience-politics.org/de/positionspapiere/

Answer 1

Found the solution. The umlauts were actually different ASCII characters than the standard ones (e.g. 097 204 136 instead of 195 164 for ä). We suspect they come from a time when the website was using an outdated LaTeX plugin. Swapping the characters for normal umlauts fixed the problem.