Two cases in .mocharc.json:

Typescript compiles, testdouble doesn't work:

"experimental-specifier-resolution=node",
"loader=ts-node/esm"

testdouble has a lengthy exception as indicated in the Git issue (same result whether changing the order, or including or excluding experimental-specifier-resolution:

"loader=ts-node/esm",
"loader=testdouble"