Just did a lot of digging. The problem is from the speedometer board:
https://www.volvoclub.org.uk/faq/Electr ... Repair.htm

Its an easy fix

I have exactly the same issue as you do - no errors. Speedo works. PNP switch is new. Checked and fixed all connectors and grounds but car shifts to second gear if I push it to 4000-5000 rpm and return the gas. Oil is new, transmission is 10-15k after rebuild. Any ideas? The car wasn't used for 10 d...