Assert in automated testing (not see elsewhere) with tools/javac/patterns/Exhaustiveness.java on linux-aarch64-debug:

src/hotspot/share/opto/type.cpp:3159),
# assert(!instance_klass->maybe_flat_in_array()) failed: cannot be flat

Current CompileTask:
C2:2281430 38732 s 4 com.sun.tools.jdi.VirtualMachineImpl::objectMirror (333 bytes)

Stack: [0x0000ffff58fa0000,0x0000ffff5919e000], sp=0x0000ffff591987f0, free space=2017k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x19c8c64] TypePtr::compute_flat_in_array_if_unknown(ciInstanceKlass*, bool, TypePtr::FlatInArray) const+0xc4 (type.cpp:3159)
V [libjvm.so+0x19ced18] TypeInstPtr::as_klass_type(bool) const+0x88 (type.cpp:4892)
V [libjvm.so+0x18ec1b8] SubTypeCheckNode::verify(PhaseGVN*)+0x634 (subtypenode.cpp:205)
V [libjvm.so+0x18ec954] SubTypeCheckNode::Ideal(PhaseGVN*, bool)+0x10c (subtypenode.cpp:149)
V [libjvm.so+0x16baa98] PhaseIterGVN::transform_old(Node*)+0xe8 (phaseX.cpp:668)
V [libjvm.so+0x16bf060] PhaseIterGVN::optimize()+0xd8 (phaseX.cpp:1054)
V [libjvm.so+0xa3b368] Compile::Optimize()+0x25c (compile.cpp:2841)
V [libjvm.so+0xa3e04c] Compile::Compile(ciEnv*, ciMethod*, int, Options, DirectiveSet*)+0x14cc (compile.cpp:880)
V [libjvm.so+0x84641c] C2Compiler::compile_method(ciEnv*, ciMethod*, int, bool, DirectiveSet*)+0x2d8 (c2compiler.cpp:149)
V [libjvm.so+0xa4cf0c] CompileBroker::invoke_compiler_on_method(CompileTask*)+0xb08 (compileBroker.cpp:2345)
V [libjvm.so+0xa4de38] CompileBroker::compiler_thread_loop()+0x638 (compileBroker.cpp:1989)
V [libjvm.so+0xfb4908] JavaThread::thread_main_inner()+0x108 (javaThread.cpp:777)
V [libjvm.so+0x1991acc] Thread::call_run()+0xac (thread.cpp:242)
V [libjvm.so+0x1624fdc] thread_native_entry(Thread*)+0x11c (os_linux.cpp:860)
C [libc.so.6+0x80b50] start_thread+0x300

Occurred post merge of jdk-27+2, only after integration, not during PR testing, and on a single platform linux-aarch64-debug (could be an intermittent issue ?).