Empty (not ended) session - freeradius 3.0.16

[dg]

Hello,

I have strange problem.

>From time to time I have session not ended (and without any user assigned to
that session).

So i have time when session starts but stop time is NULL.

example below:

SELECT radacct.RadAcctId, hotspots.name as hotspot, radacct.UserName,
radacct.FramedIPAddress, radacct.AcctStartTime, radacct.AcctStopTime,
radacct.AcctSessionTime, radacct.AcctInputOctets, radacct.AcctOutputOctets,
radacct.AcctTerminateCause, radacct.NASIPAddress FROM radacct LEFT JOIN
hotspots ON radacct.calledstationid = hotspots.mac WHERE FramedIPAddress=''
ORDER BY radacctid asc LIMIT 0, 1000; 

results

...
|      1438 | NULL    | user1      |                 | 2018-02-27 10:31:04 |
2018-02-27 10:35:33 |             269 |         1414483 |         40634419 |
Admin-Reset        | 192.168.1.110 |
|      1439 | NULL    | user2       |                 | 2018-02-27 10:31:11 |
2018-02-27 10:36:07 |             296 |         1714244 |         27071082 |
Port-Error         | 192.168.1.106 |
|      1445 | NULL    |             |                 | 2018-02-27 09:49:24 |
NULL                |            6215 |        89145682 |          1799384 |   
                | 192.168.1.110 |
|      1446 | NULL    |             |                 | 2018-02-27 09:49:46 |
NULL                |            6242 |        89124192 |          1803157 |   
                | 192.168.1.110 |
|      1447 | NULL    |             |                 | 1993-03-01 01:01:22 |
NULL                |       788783565 |        92045995 |          1920254 |   
                | 192.168.1.110 |
|      1448 | NULL    |             |                 | 1993-03-01 01:01:24 |
NULL                |       788783603 |        93859941 |          1975125 |   
                | 192.168.1.110 |


so you can see that AcctStopTime is sometimes NULL.
Can someone help me ? why its happening ?
i am using cisco switches

best regards