Custom NetSerialize for TMaps Not Compiling Anymore 4.27

In 4.26 I had followed this tutorial for serializing a TMap uproperty in a struct for RPCs calls via custom NetSerialize. However, in 4.27 the compiler seems to ignore this and throws an error, telling me that maps are not supported.
Looks to me like the compiler is ignoring the WithNetSerializer = true for uproperties.
Does anyone know how I can fix this? Is this not supported behaviour anymore?