When first enabling/verifying TOTP auth in the security pane, the input doesn't support autocomplete, preventing password managers from auto-filling it. The input appears to be missing the autocomplete HTML tag (the input in the main login flow has it).
TOTP verification flow:
<input type="text" placeholder="000000" class="code-input" inputmode="numeric">
Main login flow:
<input id="code" type="text" required="" maxlength="8" autocomplete="one-time-code" autocapitalize="characters" placeholder="6-digit code">
Obviously this is just a small papercut since generally a user would only do this once.