env vars interpolation across multiple --env-file

I have two env files:
.env1
.env2

.env1 contains FOO=foo
.env2 contains BAR=${FOO}bar

My example program.ts:

console.log(Deno.env.get("FOOBAR"))

console.log(Deno.env.get("FOOBAR"))

If I run deno run --env-file=.env1 --env-file=.env2 program.tsdeno run --env-file=.env1 --env-file=.env2 program.ts I expect to see "foobar" as the output

Instead I see barbar, which means ${FOO}${FOO} was clearly interpolated, but the value was missing. So this leads me to believe that the variables from previous env files are not available in subsequent env files.
is this true?
Is this a bug or intentionally designed this way?