It sure is the browser (Safari 12.1.2), it looks correct in Chrome. Is there a setting that I can "force" the local preview to be opened in Chrome, although it is not my default browser?
No, jAlbum will always use your default browser.
I don't normally use Safari (even on my Macbook), so I'm not familiar with it. It works OK for me, but I've never customized Safari at all - I'm using it with all default settings.
At a minimum, you should force some cache-clearing in the browser. Just hit CMD-ALT-E while you're in the browser.
But you might also have to remove extensions and plugins.
I made an fresh album with one image only, all default settings and one line of info text with a bold tag. No luck, the preview (in Safari) does NOT show any bold text, the preview in Chrome does show bold correctly. I doubt that this is caused by a faulty tag, it is a Safari problem and it will not impair my future work as long as I can get the correct results in the online version of my album or locally in Chrome.