Client Data Map data is lost for datapack registries

[SiverDX]

Minecraft: 1.21.1
NeoForge: 21.1.93

Datapack registries are created twice in this order:
First creation -> DataMapLoader#apply happens -> Second creation (previous loaded data is lost?)
(Both creations happened on the Render thread)

From Discord:

are data maps actually properly synced to the client?
in the builder i defined .sync (even with mandatory)
server has the data, client does not

in RegistryDataMapNegotiation my map is in the KnownRegistryDataMapsPayload
In ClientRegistryManager it is also present (also part of ourMandatory)

But when you retrieve a value, the innerMap returned from dataMaps in BaseMappedRegistry#getData is null
(and dataMaps is empty)

ClientRegistryManager#handleDataMapSync is never called, at least it never triggers a break point

DataMapLoader#apply seems to receive them, incl. the actual data

it's a datapack registry, i can see that the entity type registry has the parrot imitation map data still
While it's in the DataMapLaoder I can also see the entries, even when it's currently applying some other map

Breakpoints for the clear() call of the dataMaps field never triggered
I noticed though that after the whole loading the registry was a different java object - did it get re-created and the data was lost?

Yeah it seems after the data loader happens a new registry is created
https://gist.github.com/SiverDX/b53cb5fa01c005051648963d6bc2ea20