I had exactly the same problem. Neither Digitec nor Lenovo has a replacement tip available. When I asked Lenovo Support in December 2024, they told me that Lenovo does not currently offer replacement tips for the pen. He advised me to report the pen under warranty. I did this at Digitec. After 10 minutes it was credited without asking. I have now ordered and received a new one.
A nonsense without equal but there is no alternative.
I hope this helps.