As noted at https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=30362#c19 due dates with a time of 23:59 get the time erased. If the due date time is any time but 23:59, the time will appear under columns like "Due date" and "Renew". However, if it's "23:59", then the time doesn't appear.
Personally, I don't know why we wouldn't show 23:59 - at least on the staff interface. On the OPAC, I suppose a library might not want to show the exact time it's due except for hourly loans, but then we allow libraries to set specific times as well, so...
Bug 34292 got me looking at koha-tmpl/intranet-tmpl/prog/en/includes/js-date-format.inc which also has some handling for 23:59. So if there is any chagne to Koha::DateUtils, we'd also need to make it to this include.
This is by design. 23:59 is used for 'daily' due dates and hence we decided to not show the time in these cases. I think the solution for bug 30362 would be to change the phrasing in that case instead of showing the time. I think our libraries would not be so happy with the 23:59 showing everywhere and it would be more confusing than helpful to them. It also helps to have hourly loans stand out more.