I'm seeing a similar problem on Safari 14.0.3 on MacOS Catalina. If they're set to Curved or Straight, they don't show up, even on an export. Works fine on Chrome (but I prefer Safari as my mac is getting old and Chrome's performance has gone downhill on it by comparison to Safari).
Apparently this is another consequence of a known bug which will be fixed in the next update. For now to make labels appear you need to change the font size (Style > Elements > Font size) to anything.
