This is a Firefox bug in 68.0.x. It only seems to affect a gradient that is set to exactly 90 degrees in Hype’s inspector. It has already been fixed in the Firefox 69.0 beta, and it looks like this is scheduled to be released next Tuesday (Sept 3).
Linear gradients have a bit of a sordid history where their original browser-prefixed syntax used angles differently than the final spec version. I’m guessing this bug was probably caused by some sort of simplification from a developer not knowing there needed to be two paths.
Hype supports a wide range of older browser compatibility, and uses a basic check not based on a particular version for which variant to use. As browser vendors nowadays opt to use feature flags instead of prefixes, and this prefix will need to stay around indefinitely to support old syntax, we took a “if it ain’t broke don’t fix it” approach, which also reduces code size in the runtime. But clearly the prefix path is not being tested enough by at least one browser vendor, so I’ve filed a case to implement a version check and use the standard syntax. For timing, I don’t think we’d be able to release our change before Firefox’s fix in a couple fo days .
As for a workaround, if your gradient is just a static background, you could do something like add a class in the Identity Inspector for the element, and add code to the head html like:
background-image: linear-gradient(90deg, #000, #ccc) !important;