tuf be6500

[Palestro]

Model: TUF-BE6500 (CN version, marked TUF_6500)
Current stock firmware: 3.0.0.6.102_57247 (extendno: 57247-g045872c_690-gdb579_Q7MA)
buildno: 102
CPU: Qualcomm ARMv8, implementer 0x51

Issue: Cannot flash swrt firmware (3.0.0.6_TUFBE6500_R5.2.9_50023-g9428016.trx)
via web UI. Error: "Firmware upgrade unsuccessful. This may result from
incorrect image or error transmission."

Already tried:

• nvram set DOWNGRADE_CHECK_PASS=1 + commit + reboot (confirmed active, still fails)
• Downgrading to earlier official Asus firmware (3.0.0.6.102_54701) — same error
• Verified file integrity (correct size, SHA256 not provided by source but
  size matches exactly)

Is this build (57247) known to be incompatible with current swrt release?
Is there a minimum/maximum supported stock firmware version?