Have attached examples of this in action, including breakpoints showing the FDateTime::Ticks count as zero. Also showed that this is an issue for the existing GetYear/GetMonth/GetDay methods (and any method that has a FDateTime param).
The MakeDateTime methods work perfectly fine as a workaround for inline definitions.
Have tested in 4.26 preview 6 and also 4.25.3 (both from launcher).
Attached is an example where the year is defined as 2020, but the resulting year is 1; attaching debugger also shows FDateTime::Ticks value as 0.
Attached is an example where the days is defined as 10, 12 as hours, 9 as minutes, and the result is 0.0:0:0. Same issue as before, FTimespan::Ticks value is 0.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-102656 in the post.