This issue gives a STR in Outlook French.
The issue is not specific to French but it's just the language of my Office/Windows.
Update: This issue occurs also in sub-menus in other applications; Thunderbird has been confirmed.

Steps to reproduce:

1. Go in Outlook's message list and select a message
2. Press the application key to open the context menu
3. Press down arrow many times until you reach an item which has a submenu (you will hear "collapsed")
4. Press right arrow to open the submenu
5. Go to advanced options and set 'Attempt to cancel speech for expired focus events' option to 'No'
6. Re-execute steps 1. to 4.

Actual behavior:

At step 4. you can hear:
'expanded' (interrupted), 'list', 'menu'

The log is the following when pressing right arrow:

IO - inputCore.executeGesture (17:07:55.077) - winInputHook (10864):
Input: kb(desktop):rightArrow
IO - speech.speech.speak (17:07:55.167) - MainThread (13900):
Speaking ['expanded']
IO - speech.speech.speak (17:07:55.307) - MainThread (13900):
Speaking ['list', CancellableSpeech (still valid)]
IO - speech.speech.speak (17:07:55.327) - MainThread (13900):
Speaking ['Boîte de réception', 'not selected', CancellableSpeech (still valid)]
IO - speech.speech.speak (17:07:55.337) - MainThread (13900):
Speaking ['menu', CancellableSpeech (still valid)]

We can see in the log that 'Boîte de réception' and 'not selected' are spoken but they are actually not heard at all.

At step 6. you can hear:
'expanded' (interrupted), 'list', 'Boîte de réception', 'not selected', 'menu'
Note: 'Boîte de réception' stands for 'Inbox' in French and was the first item of the submenu.

The log is the following when pressing right arrow:

IO - inputCore.executeGesture (17:18:47.738) - winInputHook (10864):
Input: kb(desktop):rightArrow
IO - speech.speech.speak (17:18:47.827) - MainThread (13900):
Speaking ['expanded']
IO - speech.speech.speak (17:18:47.960) - MainThread (13900):
Speaking ['list']
IO - speech.speech.speak (17:18:47.977) - MainThread (13900):
Speaking ['Boîte de réception', 'not selected']
IO - speech.speech.speak (17:18:47.997) - MainThread (13900):
Speaking ['menu']

Expected behavior:

The first menu item's name ('Boîte de réception') should be reported when the menu is opened, whatever the setting of the option 'Attempt to cancel speech for expired events'.

Note

• This may be considered regression of NVDA 2021.1 since the advanced option was not enabled by default in 23020.4
• I imagine such a minor bug will not stop the release in any case.
• I think this issue is not specific to Outlook. I have read messages describing the same issue in submenus of Thunderbird. I have not verified it by myself however.

System configuration

NVDA installed/portable/running from source:

Installed

NVDA version:

NVDA 2021.1rc2

Windows version:

Windows 10 1809 (64-bit) build 17763.1935

Name and version of other software in use when reproducing the issue:

Other information about your system:

Other questions

Does the issue still occur after restarting your computer?

Yes

Have you tried any other versions of NVDA? If so, please report their behaviors.

2020.4 with default config: issue not present.
But 2020.4 with cancellable speech activated: issue present.

If add-ons are disabled, is your problem still occurring?

Yes

Does the issue still occur after you run the COM Registration Fixing Tool in NVDA's tools menu?

Yes