`textDocument/semanticTokens/full/delta` doesn't work

Hello, from initialize capabilities, rust-analyzer responses with full: {delta: true}, but in my tests, rust-analyzer doesn't response for delta request:

Full request:

Sending request 'textDocument/semanticTokens/full - (22)'.
Params: {
    "textDocument": {
        "uri": "file:///Users/fannheyward/src/hello-rust/src/main.rs"
    }
}

Full response:

Received response 'textDocument/semanticTokens/full - (22)' in 6886ms.
Result: {
    "resultId": "1",
    "data": [
        0,
        0,
        2,...
    ]
}

Delta request:

Sending request 'textDocument/semanticTokens/full/delta - (37)'.
Params: {
    "textDocument": {
        "uri": "file:///Users/fannheyward/src/hello-rust/src/main.rs"
    },
    "previousResultId": "1"
}

Delta response, still full response:

Received response 'textDocument/semanticTokens/full/delta - (37)' in 21ms.
Result: {
    "resultId": "2",
    "data": [
        0,
        0,
        2,...
    ]
}

The report provides no source file or test path. Start by tracing handling of textDocument/semanticTokens/full/delta and compare it with full handling; reproduce the shown previousResultId and resultId exchange, with done meaning the delta request returns the protocol's delta form rather than a full data array.