This discussion is a breath of fresh air. I know there is someone who is actually taking care of the "Go to Beginning" bug. I have the same problem with a K4 V 4.1.0 (1615370055). I use Kindlegen V2.5 (0626-3a91e28)
I tried for more than 5 days coding my HTML+OPF in a lot of different and exotic ways to get it work until I was informed about this discussion and the wonderful post#58 fixing the problem.
In my case, modifiying the MOBI is not an option as we are not sure it will not be rejected by KDP. So I hope that the test Tugger is running is successful (Bug fixed through the KDP process). And in the mid-term that Amazon will fix quickly this issue as they kwow what to do by just reading this discussion if they didn't lready know.
BTW, I hope someone informed Amazon support of the origin of the problem. If needed, I can do it.
So, Tugger please keep us informed. If it is OK, and in case Amazon doesn't fix the problem in Kindlegen or firmware, I will have the option to have 2 mobi. One to check the eBook on my kindle applying the Python script and another, with the bug, that will be fixed through the KDP processed.
Many thanks to all of you, I see suddendly the sun shine.