Dashboard actions and URL actions can both be difficult at times to truly troubleshoot without having the workbook available. Do you mind attaching a sample packaged workbook that the behavior is being experienced?
If a workbook cannot be packaged and attached, can you please take a few screenshots demonstrating the unexpected behavior.
In the meantime, here is some additional resources that can be utilized to possibly help troubleshoot and resolve the unexpected behaviors being experienced.
Creating a Table of Contents to Navigate to Other Dashboards | Tableau Software
This article below I believe might be close to the desired result that is wanted.
I feel your pain on this one. Any URL action on a dashboard will automatically change the webpage. They cannot independently control separate webpages. The workaround would be to put them on separate dashboards - not a good answer I know.
Please vote on these ideas.
And one of my own that needs some votes!