Please post your DAC definition and a basic version of your Graph

using System;
using PX.Data;
using PX.Data.ReferentialIntegrity.Attributes;
using PX.Data.BQL.Fluent;
using PX.Objects.CS;

namespace Assainissement
{
    [Serializable]
    [PXCacheName("Sanitation Customer")]
    public class SanitationCustomer : IBqlTable
    {
        #region ClientID
        [PXDBIdentity(IsKey = true)]
        [PXUIField(DisplayName = "Client ID")]
        [PXParent(typeof(Select<SanitationCustomer, Where<SanitationCustomer.clientID, Equal<Current<SanitationCustomer.clientID>>>>))]
        [PXSelector(
            typeof(Search<SanitationCustomer.clientID>),
            typeof(SanitationCustomer.clientID),
            typeof(SanitationCustomer.name),
            SubstituteKey = typeof(SanitationCustomer.clientID),
            DescriptionField = typeof(SanitationCustomer.name)
        )]
        public virtual int? ClientID { get; set; }
        public abstract class clientID : PX.Data.BQL.BqlInt.Field<clientID> { }
        #endregion


        #region BreweryID
        [PXInt]
        [PXUIField(DisplayName = "Selected BreweryID", Visibility = PXUIVisibility.Visible)]
        [PXDBDefault(typeof(SanitationCustomer.clientID), PersistingCheck = PXPersistingCheck.Nothing)]
        public virtual int? BreweryID { get; set; }
        public abstract class breweryID : PX.Data.BQL.BqlInt.Field<breweryID> { }
        #endregion  


        #region Name
        [PXDBString(80, IsUnicode = true)]
        [PXUIField(DisplayName = "Name")]
        public virtual string Name { get; set; }
        public abstract class name : PX.Data.BQL.BqlString.Field<name> { }
        #endregion

        #region Address
        [PXDBString(50, IsUnicode = true)]
        [PXUIField(DisplayName = "Address")]
        public virtual string Address { get; set; }
        public abstract class address : PX.Data.BQL.BqlString.Field<address> { }
        #endregion

        #region City
        [PXDBString(50, IsUnicode = true)]
        [PXUIField(DisplayName = "City")]
        public virtual string City { get; set; }
        public abstract class city : PX.Data.BQL.BqlString.Field<city> { }
        #endregion

        #region PostalCode
        [PXDBString(10, IsUnicode = true)]
        [PXUIField(DisplayName = "Postal Code")]
        public virtual string PostalCode { get; set; }
        public abstract class postalCode : PX.Data.BQL.BqlString.Field<postalCode> { }
        #endregion

        #region Phone
        [PXDBString(50, IsUnicode = true)]
        [PXUIField(DisplayName = "Phone")]
        public virtual string Phone { get; set; }
        public abstract class phone : PX.Data.BQL.BqlString.Field<phone> { }
        #endregion

        #region Email
        [PXDBString(50, IsUnicode = true)]
        [PXUIField(DisplayName = "Email")]
        public virtual string Email { get; set; }
        public abstract class email : PX.Data.BQL.BqlString.Field<email> { }
        #endregion

        #region EmailPreference
        [PXDBBool]
        [PXUIField(DisplayName = "Notification par courriel")]
        public virtual bool? EmailPreference { get; set; }
        public abstract class emailPreference : PX.Data.BQL.BqlBool.Field<emailPreference> { }
        #endregion

        #region Status
        [PXDBString(50, IsUnicode = true)]
        [PXUIField(DisplayName = "Status")]
        [PXStringList(new string[] { "Ouvert", "Fermé définitivement", "Fermé temporairement", "Rénovation", "Saisonnier", "Lignes non utilisées" }, new stringt] { "Ouvert", "Fermé définitivement", "Fermé temporairement", "Rénovation", "Saisonnier", "Lignes non utilisées" })]
        public virtual string Status { get; set; }
        public abstract class status : PX.Data.BQL.BqlString.Field<status> { }
        #endregion
    }
}

using PX.Data;
using PX.Data.BQL.Fluent;

namespace Assainissement
{
    public class ClientSanitationMaint : PXGraph<ClientSanitationMaint, SanitationCustomer>
    {
        public PXSelect<SanitationCustomer> SanitationCustomers;

        public PXSave<SanitationCustomer> Save;
        public PXCancel<SanitationCustomer> Cancel;

        protected virtual void _(Events.RowInserted<SanitationCustomer> e)
        {
            if (e.Row != null)
            {
                PXTrace.WriteInformation($"RowInserted: ClientID = {e.Row.ClientID}");
            }
        }

    }
}

Try replacing ClientID with this:

#region ClientID
[PXDBIdentity(IsKey = true)]
[PXUIField(DisplayName = "Client ID")]
[PXSelector(
    typeof(Search<SanitationCustomer.clientID>),
    typeof(SanitationCustomer.clientID),
    typeof(SanitationCustomer.name),
    DescriptionField = typeof(SanitationCustomer.name)
)]
public virtual int? ClientID { get; set; }
public abstract class clientID : PX.Data.BQL.BqlInt.Field<clientID> { }
#endregion

Thank you man! Worked like a charm! 

I’m not certain which thing was doing it, but ePXParent] is for associating detail records with master records, and since you don’t have a surrogate key, you don’t need to specify SubstituteKey.