I am not sure of the reason, but in the last few days, Safari has been unwilling to save my homepage and wants to replace it with Apple.com. I have changed it in the prefs a number of times with no success. I am running High Sierra after two unsuccessful attempts to upgrade to Mojave, but ran into SSD issues and went back to High Sierra until I get more time to mess with it. I trashed my Safari prefs file from my LIbrary and that did not fix it. I called Apple and they suggested restoring my Mac OS which I am hesitant to do. Apparently you cannot restore Safari from Time Machine either. I am looking for ideas. Never had an issue like this one before.
Try quoting Safari. Then hold the SHIFT key and launch Safari.
This should clear the Safari cache.
Yeah, I've seen this behaviour on multiple occasions but generally after trying a few times it manages to stick. eg. fill it in, click on another preferences tab, click back on the general tab, fill it in again, maybe repeat again. Quit Safari, try & repeat again.
I haven't managed to work out what the cause is - but the Shift key on launch as suggested above (which also prevents re-opening of any previous Safari windows/tabs/sites) is a good idea.