Fix tabs switching between tree and roadmap in Epic show page

What does this MR do and why?

Fixes a bug where switching between Epics and Issues tab and Roadmap tab within an epic page didn't work as expected.

Screenshots or screen recordings

Screen_Recording_2021-09-29_at_8.13.02_PM

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #340571 (closed)