[tlaplus] Understanding stuttering

In below Floyd's algorithm to find cycle, i see stuttering when state(a) => CycleSize=2, Fast=0, Slow=1. In next state(b), CycleSize=2, Fast=0, Slow=0. Since Slow has changed,

EXTENDS Integers,
        TLC \* for debugging

CONSTANTS N

VARIABLES CycleSize, Fast, Slow, Done, Steps

vars == << CycleSize, Fast, Slow, Done, Steps >>

TypeOK ==
    /\ CycleSize \in 1..N
    /\ Fast \in 0..CycleSize-1
    /\ Slow \in 0..CycleSize-1
    /\ Steps \in Nat
    /\ Done \in {TRUE, FALSE}

Init ==
    /\ CycleSize \in 1..N
    /\ Fast \in 0..CycleSize-1
    /\ Slow \in 0..CycleSize-1
    /\ Done = FALSE
    /\ Steps = 1
    \* different starting position
    /\ Fast # Slow

MovePointers ==
    /\ Done = FALSE
    /\ Fast' = (Fast + 2) % CycleSize
    /\ Slow' = (Slow + 1) % CycleSize
    /\ Done' = (Fast' = Slow')
    /\ Steps' = Steps + 1
    /\ UNCHANGED << CycleSize >>

Next ==
    \/ MovePointers
    \/ (Done = TRUE /\ UNCHANGED vars)

Spec ==
    /\ Init
    /\ [][Next]_vars
    \* /\ WF_vars(Next)

Termination ==
    <>[]Done