I can reproduce this with TB 91.2.0
(TB 78.14.0 works fine)

fresh profile
setup email account
account settings, create new key

compose email to a recipient, whose key can be online discovered, e.g. kaie[at]kuix[dot]de
enter something into subject and message body
options require encryption

click send
get alert that sending isn't possible (no key present). confirm

get the (level 1) popup saying "no accepted key".
click the email address line
click manage keys for recipient

get the (level 2) popup showing an empty list of keys
click discover new or refreshed key

get popup (level 3) offering import, showing fingerprint of discovered key
click ok to import

get popup (level 3) showing confirmation about imported key
click ok

Actual behavior:
now you are in a stuck state
The composer window is in the foreground, but none of its controls respond to clicks

look at the title bar of the composer window. above it there's another menu title bar, grayed out.
you may double click the title bar to zoom it (or click the green icon)
you will see that the title bar belongs to the earlier level 1 popup

This means we have a window stacking/ordering problem. The composer window was incorrectly moved to the front of the visible window stack - although there are modal popups active. The modal popups were moved to the back of the composer window.

Expected behavior (which you get with TB 78):
After clicking OK in the import confirmation dialog, you should arrive back in the level 2 popup window, and the popups should be shown on top of the composer window.

You can use any email address for testing.
Importing a key isn't necessary to reproduce this bug.

If you click discover, but no key is found, you'll get a popup saying that no key was found, and then immediately you are in the stuck state.

I cannot reproduce the bug on Linux.

I notice that on Linux, each new popup is implemented as its own separate modal dialog.

On macOS, I see that the modal dialogs are implemented differently. There is a sub-dialog appearing in the window, no separate title bar. And when a new popup opens, the earlier popup is hidden.

Looks like TB on macOS uses its own implementation for modal dialogs, which is actually broken?

I found bug 1715740 which has very similar symptoms with other Thunderbird dialogs.

I would assume that is a duplicate.