This seems to be a common issue on other forums, particularly Google, but there are no real answers so far.

Issue occurs when Samsung Smart Switch is used to move from an older Galaxy to an S25 (I have ultra, but I think it may occur with both the S25 and S25 Ultra).

Symptoms:  Location is turned on, and seems to work properly.  In Maps, you can see the blue dot moving as you move, but in Timeline, there is no NEW activity listed since the transfer from the old phone.  Timeline is turned on, Maps has needed permissions.  The only "fix" (which is not any sort of fix!) is to do a factory reset on the phone and then start over not using Smart Switch.

It seems that something Smart Switch has done with the data has somehow blocked Timeline from saving new data.