Does each role have its own separate decay timer?
It has been confirmed that each role has its own separate decay timer.
However, when you play a position, the timer is reset for that position and every position with a lower rank. Decay timer also now only applies to Diamond 4 and above.
Decay timer remains the same as last season for Master+.
Source: https://nexus.leagueoflegends.com/en-us/2019/02/ask-riot-ranked-and-more-ranked/