We use a different workflow branch factory than the default for GitHub multibranch pipeline jobs, and the branch factory sets a display name that is sometimes different than the actual branch name. When using this plugin and using getRunUrl, it uses the display name of the job instead of the actual job name, which results in non-rendering pages, etc in the UI.
I'm not sure of the proper fix here, as this may be valid for non-GH multibranch pipeline jobs. Looking at how the classic URL provider plugin handles this (following it down the chain from the plugin itself), it appears that this uses item group names, etc to get this right.
It's also possible that using getName instead of getDisplayName will just work in all cases, not sure.