I learnt that since late Sep 2022, the duration of IG stories has increased from 15 seconds to 60 seconds. However, today when I tried to post a 25-second video to Instagram Stories in my Android phone (actually the video was an MP4 converted from a still picture using ffmpeg, because there are words in the picture I would like viewers to have enough time to read), added music to it (Instagram also showed 25 seconds in this step but the number was greyed) and then published the story, the published story became 10 seconds only (not even the old limit of 15 seconds).
I tried a few times but the result was still the same. I searched the Internet but seems no one mentioned a similar problem. What steps do I have to do in order to make sure the video (and the music) can last for 25 seconds? Did Instagram detect my video was actually a still picture so it treated it as a photo?