[SBE][replica_sets] postBatchResumeToken missing after find on oplog

jsTestLog("Running initial query on the oplog");                                                                                                                                                                                                                              

{

    const res = assert.commandWorked(localDb.runCommand({

        find: "oplog.rs",

        filter: {op: "i", "o.ans": 42},

        hint: {$natural: 1}, 

        batchSize: 1,

        $_requestResumeToken: true

    }));

    assert.eq(res.cursor.firstBatch.length, 1, res);

    assert.eq(res.cursor.firstBatch[0].o._id, 0, res);

    // Assert resume token is non-null.

    const resumeToken1 = assertExpectedResumeTokenFormat(res); // <-- fails

[js_test:resume_after_against_oplog] uncaught exception: Error: Not all of the values from [ "postBatchResumeToken" ] were in {

[js_test:resume_after_against_oplog]    "firstBatch" : [

[js_test:resume_after_against_oplog]            {

[js_test:resume_after_against_oplog]                    "op" : "i",

[js_test:resume_after_against_oplog]                    "ns" : "test.resume_after_against_oplog",

[js_test:resume_after_against_oplog]                    "ui" : UUID("5efe291d-440f-4726-bd19-328987fca64b"),

[js_test:resume_after_against_oplog]                    "o" : {

[js_test:resume_after_against_oplog]                            "_id" : 0,

[js_test:resume_after_against_oplog]                            "ans" : 42

[js_test:resume_after_against_oplog]                    },

[js_test:resume_after_against_oplog]                    "ts" : Timestamp(1617146515, 17),

[js_test:resume_after_against_oplog]                    "t" : NumberLong(1),

[js_test:resume_after_against_oplog]                    "wall" : ISODate("2021-03-30T23:21:55.763Z"),

[js_test:resume_after_against_oplog]                    "v" : NumberLong(2)

[js_test:resume_after_against_oplog]            }

[js_test:resume_after_against_oplog]    ],

[js_test:resume_after_against_oplog]    "id" : NumberLong("895888238858507122"),

[js_test:resume_after_against_oplog]    "ns" : "local.oplog.rs"

[js_test:resume_after_against_oplog] } :

[js_test:resume_after_against_oplog] doassert@src/mongo/shell/assert.js:20:14

[js_test:resume_after_against_oplog] assert.hasFields@src/mongo/shell/assert.js:286:13

[js_test:resume_after_against_oplog] assertExpectedResumeTokenFormat@jstests/replsets/resume_after_against_oplog.js:32:5

[js_test:resume_after_against_oplog] @jstests/replsets/resume_after_against_oplog.js:54:26

[js_test:resume_after_against_oplog] @jstests/replsets/resume_after_against_oplog.js:8:2

[js_test:resume_after_against_oplog] failed to load: jstests/replsets/resume_after_against_oplog.js

[js_test:resume_after_against_oplog] exiting with code -3