This has never happened to me before, when I turn on the Mavic 3 Classic (and the RC remote) I wait for the satellite fix. When home point is set I take off.
But after about 30 seconds, with the drone that has traveled many meters, a new warning appears that the home point has been set (again). Why?

Because your drone set a tentative homepoint when it first got GPS location data.
But this was probably only approximate and the satellite icon on your screen would have been yellow.
When your drone got very good GPS data, the icon would have changed to white and the home point would have updated - but to where the drone was when this happened.
This is why you should wait for the sat icon to turn white before launching.

Your recorded flight data would confirm whether this is what happened.

Have a look at the replay of the flight on the app and check the GPS satellite counts.

Sean-bumble-bee Posted at 11-12 15:17
Have a look at the replay of the flight on the app and check the GPS satellite counts.

The number of sats isn't what matters.
IWhat's important is the flight controller's assessment of GPS reliability which is seen in detail in the recorded flight data or by the colour of the satellite icon on the screen.

Hi, MaxP4. Thank you for your inquiry. Captain Labroides is correct. The Home Point can be updated when the icon is white, which indicates the GNSS signal is strong, so it is indeed possible that you launched with just enough signal strength, but not at its strongest. Please take the advice to wait for the GNSS status to turn white to set your Home Point before you begin flying. Should you have any concerns or inquiries, please do not hesitate to reach us here at DJI Forum. Thank you for your continued support.

this happens to me as well. believe its just the homeport being reset more accurately when it has better connection