Trying to update windows defender from UNC path continuously fails

I'm trying to update Windows Defender (on Win 10) using definistions stored on a UNC path.

I'm setting the path the the mpam-fe.exe file like this

Set-MpPreference -SignatureDefinitionUpdateFileSharesSources \\path\to\mpam.exe

Then I'm running Get-MpPreference to verify that the path was set (it is). Once I verify that the path is correct for SignatureDefinitionUpdateFileSharesSources I run

Update-MpSignature -UpdateSource FileShares

I instantly get the error

Update-MpSignature : Virus and spyware definitions update was completed with errors.At line:1 char:1+ Update-MpSignature -UpdateSource FileShares+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ CategoryInfo          : ObjectNotFound:    (MSFT_MpSignature:ROOT\Microsoft\...SFT_MpSignature) [Update-MpSignature], CimException+ FullyQualifiedErrorId : HRESULT 0x80070002,Update-MpSignature

This failure happens almost instantly. Just to verify that the specific powershell instance can access the fileshare in question - I followed it up by just executing the mpam-fe.exe file and it worked.