I would like to suggest increasing the default log_buf_len to 128K or 256K (not the 16M that I have on my dev unit)
The later 5.x kernels boot logs are no longer complete using the default ring buffer size.
# grep log /flash/syslinux.cfg
APPEND boot=LABEL=System disk=LABEL=Storage log_buf_len=16M quiet
# dmesg | wc
961 8316 65437
# uname -a
Linux LibreELEC 5.9.11 #1 SMP Thu Nov 26 12:54:34 UTC 2020 x86_64 GNU/Linux
Happy to raise a PR either as syslinux or CONFIG_LOG_BUF_SHIFT if this makes sense.