Why not use std::memory_order_acq_rel

I am learning the code of cppcoro project recently. And i have a question.

https://github.com/lewissbaker/cppcoro/blob/master/lib/async_auto_reset_event.cpp#L218https://github.com/lewissbaker/cppcoro/blob/master/lib/async_auto_reset_event.cpp#L284

if (waiter->m_refCount.fetch_sub(1, std::memory_order_release) == 1) // #218{    waiter->m_awaiter.resume();}

using memory_order_release writting in line 218, m_refCount using memory_order_acquire flag can load the value correctly in line 284. That is OK. But fetch_sub is a RMW opertion. To read the modification in line 284 correctly, is there also need a memory_order_aquire flag? So i wonder why don't m_refCount use memory_order_acq_rel in line 218 and line 284?

return m_refCount.fetch_sub(1, std::memory_order_acquire) != 1; // #284

Thank you.