generated Tab says 20 for the lowest note

I found a weird tab in a score I was arranging.
The lowest note of the tab was twenty for some reason on one chord. This twenty was associated with the forth highest note of the chord, even though it was for the lowest string.
I noticed that that specific chord was entered slightly wrong but that did not fix it. After moving the pitch of that lowest note around a bit (I think it was specifically when I moved it above the highest note), the tab fixed it self. In this state, I could not return to the broken version. Both the actually chord and the "wrong" chord where displayed sensibly.
The score is attached.