--Relevant dmesg log--
[  396.215915] BTRFS info (device sda2): flagging fs with big metadata feature
[  396.215927] BTRFS info (device sda2): disk space caching is enabled
[  396.218876] BTRFS critical (device sda2): corrupt leaf: root=1 block=10442806968320 sl
ot=8 ino=6, invalid location key objectid: has 1 expect 6 or [256, 18446744073709551360]
or 18446744073709551604
[  396.218882] BTRFS error (device sda2): block=10442806968320 read time tree block corru
ption detected
[  396.218964] BTRFS critical (device sda2): corrupt leaf: root=1 block=10442806968320 sl
ot=8 ino=6, invalid location key objectid: has 1 expect 6 or [256, 18446744073709551360]
or 18446744073709551604
[  396.218967] BTRFS error (device sda2): block=10442806968320 read time tree block corru
ption detected
[  396.218971] BTRFS warning (device sda2): failed to read root (objectid=2): -5
[  396.219761] BTRFS error (device sda2): open_ctree failed
--End dmesg log--

old (no error on mount):
# uname -a
Linux piano 4.19.82-gentoo #4 SMP PREEMPT Mon Aug 3 20:30:11 CDT 2020 x86_64 Intel(R) Core(TM) i7-2600K CPU @ 3.40GHz GenuineIntel GNU/Linux
new kernel (with error on mount):
# uname -a Linux piano 5.15.23-gentoo #4 SMP PREEMPT Mon Aug 3 20:30:11 CDT 2020 x86_64 Intel(R) Core(TM) i7-2600K CPU @ 3.40GHz GenuineIntel GNU/Linux

On BOTH:
# btrfs --version
btrfs-progs v5.15.1

On old:
# btrfs fi show /dev/sda2
Label: none  uuid: 83c6c9ca-b8fe-4d4c-aaa8-1729f90b0824
        Total devices 1 FS bytes used 2.65TiB
        devid    1 size 2.72TiB used 2.72TiB path /dev/sda2

# btrfs fi df /home
Data, single: total=2.70TiB, used=2.64TiB
System, DUP: total=32.00MiB, used=336.00KiB
Metadata, DUP: total=10.50GiB, used=6.80GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
# btrfs fi df /var
Data, single: total=2.70TiB, used=2.64TiB
System, DUP: total=32.00MiB, used=336.00KiB
Metadata, DUP: total=10.50GiB, used=6.80GiB
GlobalReserve, single: total=512.00MiB, used=0.00B