So I do a query with this URI and body
GET /sugarcrm2/rest/v11_4/Schedulers
{"filter": [{"created_by": {"$equals": "1"}}], "fields": "id,created_by", "max_num": 1000}
And get the following record in amongst others
{"id":"c97bd87e-3f8c-11eb-be7f-080027566605","date_modified":"2020-12-16T10:52:12+00:00","created_by":"1","locked_fields":[],"_acl":{"fields":{}},"_module":"Schedulers"}
That all looks fine. If I then do
GET /sugarcrm2/rest/v11_4/Schedulers
{"filter": [{"created_by": {"$equals": "1"}}], "max_num": 1000}
The record for the previous id now contains only
{"id":"c97bd87e-3f8c-11eb-be7f-080027566605","date_modified":"2020-12-16T10:52:12+00:00","_module":"Schedulers"}
This is against a local instance, so we can provide database backups or whatever else is needed to create the problem.