In version 5.15.3, launching putty from the UI doesn't load the putty configuration with the same name. It always loads the default configuration (i.e. the behavior when no name-matching config is found). This seems to be a regression (but I'm not sure which version the regression appeared exactly). It worked fine earlier this year.
It copied '"C:\Program Files\PuTTY\putty.exe" -load "WinSCP temporary session"' to my clipboard. That's weird since I know for a fact that I have a configuration in Putty with the same name.
The session in WinSCP is in a folder, and I wasn't sure if the Putty configuration had to include the folder name as well. So I tried creating two Putty configurations with and without the folder name prefix. Neither worked.