Do you mean that when you click "New Session" button, WinSCP opens a new window, instead of opening new tab within the existing window? Or how exactly are you opening a session? Or do you mean something else? It's not clear to me what you mean by "save new sessions".

There's definitely no relevant behaviour change in 5.17.10.

From version 5.17.10 started save new sessions separatelly, it is very annoying, because i have not ended my previous session, but afetr dome time opens new session if i click icon to start session. so basically after 2 hours i have 4-5 different sessions opened in my taskbar, endeed 1.