It seems to me it's simply a matter of spaces/carriage returns, for instance, here is how it looks in my instance a custom attribute, and it works appropriately. Check your syntax
```
dn: cn=schema
objectClass: top
objectClass: ldapSubentry
objectClass: subschema
cn: schema
attributeTypes: ( 1.3.6.1.4.1.48710.1.3.1400 NAME 'scimCustomFourth'
EQUALITY caseIgnoreMatch
SUBSTR caseIgnoreSubstringsMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.7
X-ORIGIN 'Gluu - SCIM test attribute' )
objectClasses: ( 1.3.6.1.4.1.48710.1.4.101 NAME 'gluuCustomPerson'
SUP ( top )
AUXILIARY
MAY ( telephoneNumber $ mobile $ carLicense $ facsimileTelephoneNumber $ departmentNumber $ employeeType $ cn $ st $ manager $ street $ postOfficeBox $ employeeNumber $ preferredDeliveryMethod $ roomNumber $ secretary $ homePostalAddress $ l $ postalCode $ description $ title $ scimCustomFourth)
X-ORIGIN 'Gluu - Custom persom objectclass' )
```