STEPS TO REPRODUCE
1. Log into CA PPM and click the link to the New UX
2. A new tab will open to the New UX
3. While in the New UX click on the Classic PPM link
4. A new tab will open to the Classic
Existing sessions with already open tabs to the New UX or Classic PPM will be used.
A new tab is opened every time the Classic PPM or New UX links are clicked
This was analyzed under DE52478
This is Working As Design
The link that is setup from Classic to the New UX is using the external link object action which always opens in new tab which by design since it could call any 3rd party application.
The external object action operation is independent from the New UX link icon back to "Classic PPM", so this link will display in new/different tab.
In general, these are two separate and independent operations and they will display in there own unique tab.