Is it consistent or sporadic? I believe Bambu sends the sliced file via ftp on port 21, with no security similar to http and ftps exists. Might simply be your router/firewall blocking port 21, maybe try port forwarding port 21 to port 21 of the internal IP of you printer. That or find the option to enable it. Probably considered a security risk at this point using ftp, granted it’s just a sliced file.
Also, the five dollar ESP32-S3 MCU has slow wifi speeds as it is more geared towards turning smart lights on and off, not high bandwidth transfers although it should still obviously work. Why they can’t just stick a 100MB LAN port on these is beyond me. Probably a dollar a pop now in bulk.
Looks like you can connect to A1/P1 now via ftps directly to the sdcard no LAN mode needed. Need to know your access code though. Upload directly to sdcard and it starts printing, that or download videos/timelapses from the sdcard.
You can also use curl to upload file
curl --ftp-pasv --insecure -T file.gcode ftps://[ip]:990/ --user bblp:[access_code]