It was an issue that seems recurring since few years ago.
One plausible and sound solution among others were turning off "Optimize Storage" for photos/videos in iCloud settings on the iPhone
Settings > Apple ID, iCloud, Media & Purchases (the tab right under "Settings" wording, with your name and avatar) > iCloud > Photos > Optimise Phone Storage (Check or uncheck).
[My bad for not bookmarking the source that time and being unable to get back there again, will refurnish with link soon]
Long story short, it DIDN'T WORK for me.
After I did it, it still cant display visual output of the video (just black screen on the video player's frame), but audio output plays as usual. This happens to videos from various browsers (Safari, Chrome) and e-learning platform etc.
I unfortunately used the same phone as you, but I checked that there are 6GB left on my storage, which should provide enough resource to load the data?