We've seen similar issues while using Collector with SQL float / double data types.
In the ArcGIS Online web map you can configure the scale (decimals) for the field but not the precision. For example, if your SQL field data type is numeric(3,1), which is a max value of 99.9, Collector doesn't know that max. If a user were to type 999.9, Collector accepts that value and stores on the device.
From our experience, when you sync you will get an error, however the items to be synced (on the cloud icon) disappear and it looks as if the Sync was successful. The features remain on your device, however they actually never get synced to the SQL DB.
The ArcGIS Server log error associated with this issue is "Arithmetic overflow error converting real to data type numeric."