I’m having the same problem. I worked with 2nd level support and they were able to replicate the issue and said that is just how it is. Sounded like “its supposed to be that way”, but really was “oh yeah, we see the same thing - best solution ‘don’t do that’”.
Really hoping this can be fixed soon - this is a pretty serious limitation for automations I’m wanting to do. It clearly was designed in the GUI to allow a Time period that has sunset or sunrise at one end and a specific time at the other, but doesn’t allow it. Always auto changes it to Sunrise to Sunset. Rarely would I ever do a sunrise to sunset. A sunset to fixed time or fixed time to sunrise are the critical ones.