Humidity advice using tado

Hi looking for some newbie help please. I checked the app which suggested excellent outdoor air quality, the indoor humidity was at 61-5 ish so I ventilated by opening all the windows as the app suggested the room was getting stuffy and it surprised me that the humidity readings increased rapidly to 71 with an increase risk of mould warning being displayed in the app so I shut the windows and turned on my dehumidifier and levels reduced rapidly to 60. It had rained last night and the sun is now shining. Also the app suggests no open window has been detected although all have been wide open for at least 15 mins with the heating set to off as it’s summer time. Can some one give me a steer as to why this has happened or if indeed I am missing something here please? Is there a good or bad time to ventilate the home? Hope some one is reading.