Metagraphs/GraphIO: How to save graph properties to file?

Specific Domains Graphs
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1

Dear Community,
I have a graph with several properties on nodes and edges but fail to save them to a standard format, for instance, GML. Here is minimal example:

#packages
using LightGraphs, MetaGraphs, GraphIO, ParserCombinator

#create graph
g = SimpleDiGraph(2)
add_edge!(g, 1, 2)

#create MetaDiGraph & set props
mg = MetaDiGraph( g )
set_props!( mg, 1, Dict(:name=>"sue", :weight=>50) )
set_props!( mg, 2, Dict(:name=>"bob", :weight=>70) )
set_prop!( mg, Edge(1,2), :relation, 2.5 )

#save graph
savegraph("mygraph.txt", mg, GMLFormat() )

The contents of mygraph.txt is, however,

graph
[
label "graph"
directed 1
	node
	[
		id 1
	]
	node
	[
		id 2
	]
	edge
	[
		source 1
		target 2
	]
]

I cannot figure out why the properties are not stored in the file. GML supports that according to the documentation.
I also tried GEXF in which also the properties are not saved to file.
Do you have any ideas how to fix that?
Thank you in advance!
Best regards, mike_k