@Frederick-Rochette You can find instructions here in the Forum on how to install previous FW.
However, I contacted support and in order to fix the problem I did:
Installed previous FW (the one before December). Didn’t solve the issue.
Factory reset. First treadmill session after was normal but problem come back on from second run on.
Installed the latest FW version, the one released this February (the fix of the bad one from December). Initially didn’t change anything. However, several treadmill sessions later, the watch is now working correctly again (detecting the correct pace in a treadmill session).
So, no idea of what’s going on here.