If I understand correct, you want that your real URL /2020/05/281255 is returned when the URL /2020/05/281255/_/some-very-long-title is asked by the client? If all your URLs look like this, I would consider adding a rewrite rule to your .htaccess or server config file (not tested):
RewriteRule "^/(.+)/_/.+" "/$1" [PT]
The passthrough option (PT) should make your clients still show the long URL in the browser - if this is something you want.
well, somehow the RewriteRule with the [PT] flag doesn’t seem to work and will result in a page not found error meaning Kirby is showing the error page. If I read the Apache documentation correctly those kind of redirects only seem to work with file paths:
I do not think, that this really matters but maybe the correct placement together with the other rewrite rules from kirby might have an effect. But - as I wrote - I didn’t test this together with Kirby, but the rewrite rule as such should be ok.