Cannot customize menu appearance from Upfront Builder

Hello,
I'm using Upfront Issue theme.
Whenever I try to custom the appearance of my top bar menu, via Upfront Builder, clicking on the menu, then cog icon, then Appearance section, it always revert back to default.

If I add a new Preset appearance, it is applied and it seems fine, but when I click on SAVE THEME, although I see it correctly in Upfront Builder, it is not displayed correctly (that is with my customized style) live on my site.
Refreshing Upfront Builder, I see the menu indeed back with the old style.

Same thing happens if I edit the DEFAULT NAVIGATION appearance, or any other appearance style.

Please help, thanks!

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