When I try btrfs check/repair/rescue (in the below case check) /dev/sda6 (my root partition, from 12sp2 rescue) I get:

Checking filesystem on /dev/sda6
UUID: eac3aac4-d03a-4edd-b91f-a857bdb16226
checking extents
ctree.c:1612: leaf_space_used: Assertion 'data_len < 0' failed.
btrfs[0x43e3dd]
btrfs[0x43e94c]
btrfs(btrfs_leaf_free_space+0x2e)[0x43ffde]
btrfs(btrfs_check_leaf+0x9d)[0x4400cd]
btrfs[0x428215]
btrfs[0x429b8d]
btrfs[0x42a8ee]
btrfs(cmd_check+0x8e7)[0x42cc17]
btrfs(main+0x82)[0x40b742]
/lib64/libc.so.6(__libc_start_main+0xf5)[0x7f7e0d1c26e5]
btrfs(_start+0x29)[0x40b849]

Anyone have any bright ideas how to fix that?
Seems my root partition is screwed up somehow and is only mounted RO, so I tried the rescue system but doesnt seem to work.