Just a really wild guess because you have not given much description yet... But the mention of Skype and open apps might be the clue. Is this something you notice because the app is open and in use as you leave WiFi coverage, and you notice a break in service? There might be a short delay when you get to the point where WiFi still exists but the signal is too weak to sustain data transfer, before the signal drops completely and routing shifts to cellular. This would probably be more noticeable in apps with heavy continuous/real-time data like Skype but you might not notice it so easily in something like email. See if that gives you any clues.
There is also a setting under Settings/ Cellular (Mobile) Data for WiFi Assist which allows connectivity to fall back to cellular when WiFi is poor rather than waiting for a complete dropout.