I did review the new server configuration. Also, I'm on Android 11, MIUI 12.5.5 by xiaomi.eu.The ability to write to external SD card should not have been affected but the configuration of the default folders is a bit trickier now. Have you reviewed the descriptions of the server configuration, particularly default folders? Also some more details would be helpful such as the Android version, ROM variant, and root status. Also, can it be configured on internal storage or anywhere for that matter? There other things which could interfere with writing to external storage which is why more details and a log would be helpful and efficient.
Note: If you are using using external storage as internal (adopted storage) these suggestions may not apply.
I didn't think to send a log because I thought it was either a limitation from the new version or something on WinSCP side. After you asked for it, I first checked it myself and found out my SD card was read-only for some reason.
I just reformatted it and now MiXplorer is writing over FTP as it should, thanks!