Tested and works. Thanks for the quick debugging and for the fix 🙇

Works in manual testing*, but it looks like test_translations_are_applied and test_merges_child_theme_json_into_parent_theme_json php unit tests are failing

I think it's something to do with the call to WP_Theme_JSON_Resolver_Gutenberg::get_theme_data() and the status of null === static::$theme in each test. Maybe it's referencing the core instance (?)

*I tested by trying to apply blockGap to a block in theme.json, e.g.,

	"styles": {
		"blocks": {
			"core/social-links": {
				"spacing": {
					"padding": "100px",
					"blockGap": "100px"
				}
			},
			"core/columns": {
				"spacing": {
					"blockGap": "100px"
				}
			}
		}
	},

Before this change, blockGap did not appear at all.

Thanks for looking into a fix @scruffian!

I managed to get the unit tests passing for me locally if I add the following to lib/init.php:

add_action( 'switch_theme', array( 'WP_Theme_JSON_Resolver_Gutenberg', 'clean_cached_data' ) );

In core, switching a theme resets the resolver cache, which is required for these tests to pass (this is the line in core: https://github.com/WordPress/wordpress-develop/blob/b316c8b25fc71920d89ee37ee26609a77b2a305a/src/wp-includes/default-filters.php#L348). So, for the sub-classed version of static::$theme to be cleared, it looks like we need that additional action, too.

I'm wondering if this means we could encounter other edges cases as well, as there's also an action for start_previewing_theme to clear the cache, too (here)...

@scruffian based on an idea @talldan came up with, we've opened up an alternative PR in #42756 that updates the cache check instead of adding in a new static variable in the sub-classed version. It appears to still fix the issue, and works around having to add in additional actions to clear the cache.

Thanks so much for working out where the issue was!