I can replicate iPadOS 26.4.2 Calendar time-setting issue that you describe. I also note that the issue is not present with iPhone running iOS 26.4.2.
Anyone that encounters this issue would be well advised to submit a Bug Report to Apple via its Product Feedback portal. For iPad and iPadOS:
Feedback - iPad - Apple
Every report counts - as until Apple receive a sufficient number or reports, it is perhaps unlikely that resources will be assigned to investigate and resolve the issue. Please don't rely upon others submitting a similar bug report; submitting a Bug Report yourself is the only way to be certain that Apple have been made aware of the problem.
When submitting a report, ensure that you include as much detail as possible within the body of the report - including your iPad model and installed version of iPadOS; don’t rely upon the pick-list, as this rarely includes the full range of device models or most recent versions of iOS/iPadOS.
Workaround
Until resolved by Apple, there is no fix - but there is a workaround. Instead of attempting to edit hours and minutes using the numeric keyboard, highlight the time field - and then simply swipe up or down to adjust the hours and minutes as needed.
Many users may not have noticed the bug that you have identified as - on many cases - they will already be setting the time using the swipe method in preference to the keyboard.