My default font was Helvetica Neue, and changing it to Times New Roman didn't help. If it's a Chromium issue, but the linked Chromium report you mentioned hasn't had much in the way of updates, is this a Chromium issue or a macOS issue? It seems to have been going on for several years, so it doesn't seem to be tied to specific OS versions (I'm on macOS Sonoma, 14.4), and I confirmed that all the fonts listed in Vivaldi are activated.
Additionally, I don't have this problem on Edge or Chrome, so if it's a Chromium issue, how could it be impacting some of the browsers, but not others?