optional string property in model does not work properly with "exists" where clause

Steps to reproduce

create a model with an optional string property i.e.
@property({ type: 'string', }) c_modelCode?: string;

when using "exists" on this property, doesn't return any data even when there is data.
"where" : { "c_modelCode": {"exists" : true} }

if the property is removed from the model definition, data is returned with the above where filter.

Current Behavior

no data returned with "exists" clause even though data exists

Expected Behavior

return all objects with c_modelCode propery.

Link to reproduction sandbox

Additional information

+-- @loopback/authentication@7.3.1
+-- @loopback/authorization@0.9.1
+-- @loopback/boot@3.4.1
+-- @loopback/context@3.17.0
+-- @loopback/core@2.16.1
+-- @loopback/openapi-v3@5.3.1
+-- @loopback/repository@3.7.0
+-- @loopback/rest@9.3.1
+-- @loopback/rest-explorer@3.3.1
+-- @loopback/service-proxy@3.2.1
+-- loopback-connector-mongodb@6.0.1

Start by reproducing the optional string property query against the listed loopback-connector-mongodb version, then trace how the repository handles the "exists" filter for declared model properties. Done means records containing c_modelCode are returned and a regression test covers the optional-property case.