When the validate function returns an object where a key, e.g. OPT, is undefined, assignVariablesToProcess will assign undefined to process.env.OPT which will stringify it (see https://nodejs.org/api/process.html#processenv). This results in ConfigService.get returning process.env.OPT which now has the value 'undefined'.
yarn run start and observe the log.
The second generic parameter (WasValidated) changes the return value of ConfigService.get. If true, it removes undefined from the return type. If false, it adds undefined. The problem here is that a value could be both validated and undefined.
Observe the TypeScript return type of optValue in main.ts.