SharePoint User Permissions automatically get revoked

 I have faced an issue that user-assigned permissions are removed automatically in Sharepoint Permission page.

There is a SID mismatch on SharePoint Profile and FIM.

Get the $user = Get-SPuser -Web 'URL'  -identity 'i:0#.w/<domain/userid>'

if the $user return Null referencen

Please run the below script

Move-SPUser -identity $user -NewAlias ' i:0#.w/<domain/userid>' - IgnoreSID.