You must be very careful while applying any solution because sometimes many forum members are not able to answer the question but still they answer it anyhow. So, It's important to confirm the solution and then apply it.
Is disk logging enabled on both devices?
System events are configured to be logged?
On the log view page, is the right source of logs selected?
Because, since you know it's logging the information properly, as you can see on that other device, it seems to be just an viewing issue.
You might have to format the fortigate's disk, which will cause you to lose the logs you already have.
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