Dropbear SSH server does not recognize some user accounts.

Dropbear SSH does not recognize authorized_keys placed in ~/.ssh/ in user's home directory

Edit /etc/passwd with sudo nano /etc/passwd or sudo vipw, find the user, check the last section of the line (each section is seperated by comma ":"), change it from "/bin/false" to "/bin/sh" or "/bin.bash" depending on your system (Just check the other lines and use the same one). I guess "/bin/false" implies the user has no need of terminal console and hence is not registered in dropbear.

Each line of the /etc/passwd file contains seven comma-separated fields:

user:x:1001:1001:user,,,:/home/mark:/bin/bash

user:x:1001:1001:user,,,:/home/mark:/bin/bash
[--] - [--] [--] [-----] [--------] [--------]
|    |   |    |     |         |        |
|    |   |    |     |         |        +-> 7. Login shell
|    |   |    |     |         +----------> 6. Home directory
|    |   |    |     +--------------------> 5. GECOS
|    |   |    +--------------------------> 4. GID
|    |   +-------------------------------> 3. UID
|    +-----------------------------------> 2. Password
+----------------------------------------> 1. Username

For more details, go check https://linuxize.com/post/etc-passwd-file/