Movie files (.mp4) don't consistently play in Safari

URL of experiment: https://pavlovia.org/abbeyjoy/avspeechtask

Description of the problem: In each trial in this experiment, a video (.mp4) file and an audio file play simultaneously. We thought everything was working but noticed last week that in the Safari browser on MacOS, the movies don’t consistently play; often they just freeze on the first frame. In Chrome and Firefox everything seems to work fine. We recruited some coworkers to do some testing, and the problem only occurred on Macs running the High Sierra and Mojave versions of the OS–on Catalina, the videos consistently played correctly. Does anybody have an idea what might be causing this and if there’s a way to fix it (other than telling participants not to use Safari)?

Thanks in advance!

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