Forerunner 945: GPS fails only once after battery recharge

Since January 2024 I have this constant pattern:

1) I charge the phone

2) I go for a run

3) GPS works for 1km more or less then it position me hundreds of meters away

4) I stop the activity

5) I immediately start a new one and the GPS works perfectly for every successive run until I charge the phone again

I have been doing also long runs 24, 31Ks and as long as they are not the first run after the recharge they are great.

I have done a factory reset and the pattern is still there. It sounds like a software thing to me, as if it was hardware how would I get all runs tracked perfectly after the one error ?