There is no downloadable ISO of Insider Preview Build 14372 available, as far as I can determine.
A comparison - two versions of Windows 10 Pro 64bit:
Win10 Pro Build 10586
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run - No listing for Windows Defender
HKLM\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Run - No listing for Windows Defender
Settings > Windows Defender: Real-time protection - slider is ON; Cloud-based protection - slider is ON; Automatic sample submission - slider is ON.
Open Windows Defender > PC Status: Protected; Real-time protection: ON; ...definitions: Up to date.
Administrative Tools > Services:
Windows Defender Network Inspection Service: Status - Running; Startup Type - Manual
Windows Defender Service: Status - Running; Startup Type - Automatic
Win10 Pro Insider Preview Build 14372.rs1
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run - Windows Defender REG_EXPAND_SZ "D:\Program Files\Windows Defender\MSASCuiL.exe" (File exists in this location)
HKLM\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Run - no listing for Windows Defender
Open Windows Defender > PC Status: Windows Defender Real-time protection is OFF.
Home: Windows Defender is periodically scanning your PC for threats.
Windows Defender Real-time protection is off because you are using another AV program. Uninstall your existing AV program to turn on Windows Defender.
Real-time protection: Handled by another AV program
Virus and spyware definitions: Up to date
Turn On button: get message about periodic scanning
The above is what is causing the problem: First, I do not have another AV program installed, I have no idea as to what other AV program is being detected unless, somehow, the other version of Windows Defender (in my multi-boot system) is being detected.
If I check Settings > Update & security > Windows Defender: Limited periodic Scanning - slider is ON; Real-time protection - slider is greyed out - can't be moved; Cloud-based Protection - slider is ON; Automatic sample submission - slider is ON; Enhanced
notifications- slider is ON
Administrative Tools > Services:
Windows Defender Advanced Threat Protection Service: Status - blank; Startup Type - Manual
Windows Defender Network Inspection Service: Status - blank; Startup Type - Manual
Windows Defender Service: Status - Running; Startup Type - Automatic