Row store table with inconsistency 'invalid length value'

Hello,

we are doing regular consistency checks in our HANA databases,
with this SQL command:
CALL CHECK_TABLE_CONSISTENCY ('CHECK', NULL, NULL);

This went fine for a few months; but now in one of our systems we are getting errors for table SYS.CS_JOIN_CONDITIONS_:
SYS;CS_JOIN_CONDITIONS_;JOIN_NAME;?;5.995;Found invalid length value;sid=0x7fadc02fc618

I did a bit of analysis:
Table SYS.CS_JOIN_CONDITIONS_ has Type Row Store.
Column JOIN_NAME has SQL Data Type NVARCHAR,
Dimension 256,
Column Store Data Type String

Now in my understanding field JOIN_NAME could not be longer than 256 characters,
but surprisingly (for me) it can.
There are 31 entries with lengths from 258 to 454.

I also looked in a few other HANA systems:
The values in column JOIN_NAME in table SYS.CS_JOIN_CONDITIONS_ always are shorter than 256 charactes there.

Any hint what is going on here?
Or at least how to bring back consistency?

Btw, version 1.00.68.384084

kind regards