How to prevent unwanted DBG_FMMDM spam from filling up your logcat buffers
Yes, I have seen this before. It is a security feature and it is coming from Samsungs Find My Mobile {FMM}(com.samsung.android.fmm). Samsungs version of Google's Find My Device. 

It is located at:

Settings --> Lock screen (maybe Biometrics) and security --> Find My Mobile.

If the service is not wanted it can be deactivated.

I/DBG_FMMDM is short for Debug _ Find My Mobile Device Manager. 

A breakdown of what log you posted states: 

The FMM app is started but when it tries to get your Device ID it cannot find the IMEI for GSM and the MEID or ESN for CDMA phones so its returning null. However, when it checks to see if a Samsung account is connected to the device it returns positive.