Partial of a mapped type from a union of enums fails when these enums have common members

TypeScript Version: 4.0.5, 4.1.2, 4.2.0-dev.20201126

Works fine with TS 3.X

Search Terms: mapped type enum partial

Code

enum EnumA {
    A = 'A',
    B = 'B',
}

// A second enum with at least one key also in EnumA
enum EnumB {
    B = 'B',
    C = 'C',
}

type Mapped = {
    [k in EnumA|EnumB]: string;
};

const partial: Partial<Mapped> = {
    [EnumA.B]: 'value',
};

Expected behavior:
Should compile

Actual behavior:
Typescript complains that EnumA.B does not exist in Partial<Mapped> when declaring the partial variable:

Type '{ B: string; }' is not assignable to type 'Partial<Mapped>'.
  Object literal may only specify known properties, and '[EnumA.B]' does not exist in type 'Partial<Mapped>'.

Playground Link: https://www.typescriptlang.org/play?ts=4.2.0-dev.20201126#code/KYOwrgtgBAouEEEoG8BQUNSQXigcgTwBp1MAhKXPM41AX1VVElngrUygqppM4GFK+frQaoALgE8ADsCgBZAIbTZAEyEdMAbQDWUAJYhWkBAB84kMgF0AXFADO4gE6GA5gG56n1AGMA9iCOUADu+uIAFn5g4vx+EBABAGqKADZgwHYACopO4vqpADxKKsCqAHwapBhaFogAdAi2+LScNWx1-E14ot7+geJQ-vFJqekAYk5xCFk5eYXFahW4mtW1CHXWdt0kdL0BQUMJIMlpwBNxZDO5+SlFyouVrbVkG109jH1BoRFR4tnXqTsC1Kj20awaXXWhD4YPg602+HhLVhlleWxevCqUDaqM66I67yAA

Related Issues: maybe https://github.com/microsoft/TypeScript/issues/37859 ?

Begin with the linked TypeScript Playground and the supplied enum, mapped-type, and Partial repro, then compare the reported behavior across the listed TypeScript versions. Done means the example compiles as expected when the mapped type is built from the union of enums, with regression coverage for the shared enum member case.