TypeError: Cannot read property 'map' of undefined - Reply functionality broken

Nextcloud 20.0.9
Mail app 1.9.5
PHP 7.3.27-9+0~20210227.82+debian10~1.gbpa4a3d6 (cli)
Browser: Google Chrome Version 90.0.4430.212
Installed mail, connected it to my account. Mail fetched just fine, but when I press reply, all I get is blank area at the part of the screen I would expect the editor to be in. Checking with Developer tools, I see two counts of

TypeError: Cannot read property 'map' of undefined
    at Composer.vue:384
    at Array.flatMap (<anonymous>)
    at aliases (Composer.vue:374)
    at pn.get (vue.runtime.esm.js:4479)
    at pn.evaluate (vue.runtime.esm.js:4584)
    at a.aliases (vue.runtime.esm.js:4836)
    at a.<anonymous> (Composer.vue?323e:1)
    at a.t._render (vue.runtime.esm.js:3548)
    at a.r (vue.runtime.esm.js:4066)
    at pn.get (vue.runtime.esm.js:4479)

I wonder if there is anything I can do to resolve this?

This does not happen with firefox. Should I file a bug report?

yes please

I can no longer reproduce the error now that I’ve upgraded to Nextcloud 21.0.1.
(Actually the upgrade from 20.0.9 to 21.0.1 solved some other minor strange items too)