Hello @ianniow and welcome to the forum!
I’m not sure if you can see those past warnings now, but if reproducible, you can watch the live server log with the level dropdown set to Warning. About → Show log → Live → Warning. If you want an always-on log you could instead set up a –logfile. You don’t even need –log-file-log-level set if you want Warning level.
There’s some chance that a reporting option such as emails on non-successes will send you the relevant lines. Email seems able to send out material that isn’t normally stored, unless you asked for a log yourself.
I’m not sure how the 25 count is calculated, but there are unfortunately several places to look for warnings. Job → Show log → General (which is where the button to see warnings goes) and About → Show log → Stored (which generally has less, but sometimes if the other log has nothing, it’s a good second log to try).
Some of the results presentation has been rewritten recently (nothing in beta yet, but in canary for awhile). Improve UX when dealing with backup logs discusses various issues, e.g. how many warnings to show… Improve UI for backup logs #3510 has some screenshots. Note that canary builds can be bleeding-edge…