Scheduling a workflow trigger on GitHub Actions isn’t exactly the most reliable method, especially if you’re on the free tier. I’ve been using event-based like push, pull_request and manual trigger without any problems, but scheduling has definitely been a bit tricky.
There are some known constraints, like if your public repo hasn’t had any activity for the last 60 days, the scheduled workflow will only trigger on the default branch, and your workflow needs to be in the default branch as well. That’s clear enough, but then there’s this:
“The schedule event can be delayed during periods of high loads of GitHub Actions workflow runs. High load times include the start of every hour. If the load is sufficiently high, some queued jobs may be dropped. To decrease the chance of delay, schedule your workflow to run at a different time of the hour.” – Source (official docs)
This explanation seems vague.
In the past, some people have tried downgrading their version to fix this issue, and it was also suggested that the shortest interval should be avoided.