I noticed this bug in the past and now it happened again:
You can check for new versions in 2 ways:
1. Clicking the button on the button bar at the right side ("Shows information about application updates")
2. From Help -> Check for updates.
However, the behavior is not consistent.
I just opened WinSCP and clicked the button. I got a message saying "You have the latest version".
Then I went to Help-> Check for updates and this time I was informed that "New version 5.5.2 was released"
Please see attachment with screenshots.