Do you expect your CL may affect the flow of download UI?

Definitely not, at least in the case of my CL:

https://luci-milo.appspot.com/buildbot/tryserver.chromium.win/win_chromium_rel_ng/515758
https://chromium-review.googlesource.com/c/chromium/src/+/622068

kbr -- is this test flaky or failing reliably?
Do we know when it started failing?

Please search through the recent builds on this tryserver:
https://luci-milo.appspot.com/buildbot/tryserver.chromium.win/win_chromium_rel_ng/?limit=200

It's flaky. Currently there are several incidences visible. These aren't necessarily an exhaustive list:
https://luci-milo.appspot.com/buildbot/tryserver.chromium.win/win_chromium_rel_ng/526490
https://luci-milo.appspot.com/buildbot/tryserver.chromium.win/win_chromium_rel_ng/526473
https://luci-milo.appspot.com/buildbot/tryserver.chromium.win/win_chromium_rel_ng/526448
https://luci-milo.appspot.com/buildbot/tryserver.chromium.win/win_chromium_rel_ng/526401
https://luci-milo.appspot.com/buildbot/tryserver.chromium.win/win_chromium_rel_ng/526371

Here's the log excerpt from one of the failing runs:

[ RUN      ] DownloadTestWithFakeSafeBrowsing.SendUncommonDownloadReportIfUserProceed
[3760:2992:0905/100607.867:ERROR:direct_composition_surface_win.cc(1095)] Failing to detect HDR, couldn't retrieve D3D11 device from ANGLE.
[3760:2992:0905/100607.871:INFO:media_foundation_video_encode_accelerator_win.cc(335)] Windows versions earlier than 8 are not supported.
[5096:3540:0905/100608.165:INFO:chrome_cryptauth_service.cc(222)] Profile is not authenticated yet; waiting before starting CryptAuth managers.
[5096:3540:0905/100608.166:ERROR:event_page_request_manager.cc(154)] An error encountered while waking the event page.
[5096:3540:0905/100608.166:ERROR:event_page_request_manager.cc(96)] Draining request queue. (queue-length=3)
[5096:3840:0905/100608.223:FATAL:download_file_impl.cc(243)] Check failed: (sequence_checker_).CalledOnValidSequence().
Backtrace:
	base::debug::StackTrace::StackTrace [0x029E3667+55]
	base::debug::StackTrace::StackTrace [0x029A458A+10]
	content::DownloadFileImpl::~DownloadFileImpl [0x01CE5183+98]
	content::DownloadFileImpl::`scalar deleting destructor' [0x01CE54AB+11]
	base::internal::BindState<<lambda_e3f6ba5a3254185064e39016a63e6e1c>,base::internal::PassedWrapper<std::unique_ptr<media::VideoCaptureBufferHandle,std::default_delete<media::VideoCaptureBufferHandle> > > >::~BindState<<lambda_e3f6ba5a3254185064e39016a63e6e [0x01CE9C58+16]
	base::internal::CallbackBase::~CallbackBase [0x02968468+24]
	base::internal::SchedulerWorker::Thread::ThreadMain [0x02A0E42B+411]
	base::PlatformThread::Sleep [0x02979942+290]
	BaseThreadInitThunk [0x7590337A+18]
	RtlInitializeExceptionChain [0x773E9882+99]
	RtlInitializeExceptionChain [0x773E9855+54]