Trump himself endorsed McCain, Ryan, Romney, Collins, Sasse, and many other Republicans who had previously trashed him.
Was DeSantis supposed to endorse O'Dea's Democrat opponent (as Trump himself did in Georgia's gubernatorial race)?
There were races where DeSantis didn't endorse anybody. For example DeSantis didn't endorse anybody in the GA Senate race.
DeSantis chose to endorse Bush establishment O'Dea while he didn't endorse Walker in GA.
BTW, Trump mock-endorsed Abrams.