I've just spent far too long debugging, what is essentially the following code:

#!/usr/bin/env python
try:
    import asyncio
except ImportError:
    import uasyncio as asyncio

loop = asyncio.get_event_loop()

async def foo():
    print('foo()')

async def main():
    loop.run_until_complete(foo())
    while True:
        print('app_loop()')
        await asyncio.sleep(1)

if __name__ == '__main__':
    loop.run_until_complete(main())

I know what you're thinking.... why run_until_complete inside an async function.
I completely agree, and now that I have the root cause, it's easy to fix.

However: this was nested inside a relatively large code-base, and I had no trace-back leading me to the root cause.
Finding this required a lot of cutting code until the problem went away, which wasn't helped by it being in 2 places.

Run on CPython

When run on Python 3.9.9, it raises.

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/usr/lib/python3.9/asyncio/base_events.py", line 647, in run_until_complete
    return future.result()
  File "<stdin>", line 2, in main
  File "/usr/lib/python3.9/asyncio/base_events.py", line 623, in run_until_complete
    self._check_running()
  File "/usr/lib/python3.9/asyncio/base_events.py", line 583, in _check_running
    raise RuntimeError('This event loop is already running')
RuntimeError: This event loop is already running

Run on micropython

When the above code is executed in micropython I get the following output.

MicroPython v1.17 on 2021-09-02; PYBD-SF6W with STM32F767IIK
Type "help()" for more information.
>>> 
paste mode; Ctrl-C to cancel, Ctrl-D to finish
=== # ... above code pasted here
=== 
foo()
app_loop()
>>> 

Execution took 1 second, no exception raised.