Do you have a reproducible example? I cannot reproduce:
julia> using JuMP
julia> import SCIP
julia> model = Model(SCIP.Optimizer);
julia> @variable(model, 0 <= x[i = 1:4] <= i, Int)
4-element Vector{VariableRef}:
x[1]
x[2]
x[3]
x[4]
julia> @variable(model, n, Int)
n
julia> @objective(model, Max, sum(x))
x[1] + x[2] + x[3] + x[4]
julia> @constraint(model, [n; x] in MOI.CountBelongs(1 + length(x), Set([2, 3])))
[n, x[1], x[2], x[3], x[4]] ∈ MathOptInterface.CountBelongs(5, Set([2, 3]))
julia> optimize!(model)
presolving:
(round 1, fast) 6 del vars, 3 del conss, 0 add conss, 2 chg bounds, 0 chg sides, 0 chg coeffs, 0 upgd conss, 3 impls, 8 clqs
(round 2, fast) 7 del vars, 3 del conss, 0 add conss, 2 chg bounds, 4 chg sides, 1 chg coeffs, 0 upgd conss, 3 impls, 8 clqs
(0.0s) running MILP presolver
(0.0s) MILP presolver (2 rounds): 1 aggregations, 2 fixings, 0 bound changes
(round 3, medium) 10 del vars, 9 del conss, 4 add conss, 2 chg bounds, 4 chg sides, 1 chg coeffs, 0 upgd conss, 3 impls, 6 clqs
(round 4, exhaustive) 10 del vars, 9 del conss, 4 add conss, 2 chg bounds, 4 chg sides, 1 chg coeffs, 4 upgd conss, 3 impls, 6 clqs
(round 5, fast) 10 del vars, 9 del conss, 4 add conss, 2 chg bounds, 4 chg sides, 3 chg coeffs, 4 upgd conss, 3 impls, 6 clqs
(round 6, medium) 12 del vars, 11 del conss, 6 add conss, 2 chg bounds, 7 chg sides, 9 chg coeffs, 4 upgd conss, 3 impls, 6 clqs
(round 7, fast) 12 del vars, 13 del conss, 6 add conss, 2 chg bounds, 7 chg sides, 9 chg coeffs, 4 upgd conss, 3 impls, 6 clqs
presolving (8 rounds: 8 fast, 4 medium, 2 exhaustive):
19 deleted vars, 15 deleted constraints, 6 added constraints, 2 tightened bounds, 0 added holes, 7 changed sides, 9 changed coefficients
3 implications, 0 cliques
transformed 1/1 original solutions to the transformed problem space
Presolving Time: 0.00
SCIP Status : problem is solved [optimal solution found]
Solving Time (sec) : 0.00
Solving Nodes : 0
Primal Bound : +1.00000000000000e+01 (1 solutions)
Dual Bound : +1.00000000000000e+01
Gap : 0.00 %