SharePoint designer custom form link not working properly when added in quick launch

I am using SharePoint Online classic team site. I have created a custom form with the help of SharePoint Designer 2013 that helps me to add item in SharePoint List. I have added the link of that custom form in the quick launch menu of the site. The link works fine when I open it in the new tab of the browser, but when I open it in the same tab it navigates me to the list rather than form that I have added it in the link.

I have also used a custom JS file to hide some fields or for custom formatting purpose within the form. Have also checked the links on the browser tabs i.e. same and new tab. I found out the difference that the link opened in the new tab is fine, but the link in the same tab has the &viewpath= link to the form appended.

for example –

• link in new tab – https:\myspsite.comsitessitenamelistlistnamemyform.aspx
• link in same tab –
  https:\myspsite.comsitessitenamelistlistnamemyform.aspx&viewpath=sitessitenamelistlistnamemyform.aspx

I tried searching the issue on google, but I didn’t find any solution.