Check_mk-zfsget wrong size

On the FreeBSD server:

Trying 127.0.0.1... 
Connected to localhost.
Escape character is '^]'.
<<<check_mk>>>
Version: 1.6.0p14
AgentOS: freebsd
[SNIP]
<<<zfsget>>>
MyBigPool/MyDataSet     name    MyBigPool/MyDataSet     -
MyBigPool/MyDataSet     quota   1649267441664   local
MyBigPool/MyDataSet     used    1649267482528   -
MyBigPool/MyDataSet     available       0       -
MyBigPool/MyDataSet     mountpoint      /mnt/MyBigPool/MyDataSet        default
MyBigPool/MyDataSet     type    filesystem      -

So MyDataset is 1,5 TiB and full. However checkmk is sure it is only 500GB

OMD[mysite]:~$ check_mk --debug -npv 'freenas' | grep MyDataSet
Filesystem /mnt/MyBigPool/MyDataSet OK - 36.62% used (183.08 of 500 GB), (warn/crit at 100%/100%), trend: -672.42 GB / 168 hours (/mnt/MyBigPool/MyDataSet=187468.999283;514689.564472;514689.564472;0;512000 fs_size=512000;;;; growth=0;;;; trend=-98365.050446;;;0;3047.619048 trend_hoursleft=-1;;;;)

This happened spontaneous, until yesterday checkmk reported 1,5TiB. Any thoughts?

Removing some files to make used < quota fixed the problem;

MyBigPool/MyDataSet     name    MyBigPool/MyDataSet     -
MyBigPool/MyDataSet     quota   1649267441664   local
MyBigPool/MyDataSet     used    1649267117040   -
MyBigPool/MyDataSet     available       324624       -

Seem like the plugin does not handle used > quota well.