I found the reason.
The following HTML was generated:
<div id="com_users_twofactor_totp" style="display: none">
You should investigate the possibility of selecting "Google Authenticator" from the pull-down menu and still having style="display: none".
Postscript:
T4 is outputting a different source than Cassiopeia. Compare below;
Cassiopeia:
<select id="jform_twofactor_method" name="jform[twofactor][method]" onchange="Joomla.twoFactorMethodChange();" class="form-select">
Change from <div id="com_users_twofactor_totp" class="hidden">
to <div id="com_users_twofactor_totp" class>
T4:
<select id="jform_twofactor_method" name="jform[twofactor][method]" onchange="Joomla.twoFactorMethodChange()" style="display: none;">
Operating the pulldown does not change the following sources;
<div id="com_users_twofactor_totp" style="display: none">