Our team received a lot of requests regarding the custom CSS metabox plugin and custom JS metabox plugin do not work well with the Gutenberg block editor. The great news is that we patched both plugins today and they should be working correctly with Gutenberg now. If you have any questions, please let our team know.
There is a possibility that if the page takes too long to load, the saved code might not be loaded when the page loads. If that happens, try refreshing the page.