Have noticed this for a long time already.
The isobare lines disappear aren't shown on the Weather radar or Satilite maps, that's fine for me.
When switching the another view (e.g. Wind, Temperature, Clouds) afterwards these lines still are not shown.
Reloading the page doesn't help but a fresh start via www.windy.com solve this small problem.
Can see this on Windows 10 using firefox on a daily basis.