Tie direction for chords on tab staves non-deterministic?

Ubuntu Studio 14.04, GIT commit: 16c2771

Sorry, the nature of this makes it hard to be specific. But I find that a simple major chord - C E G C played bottom -to-top on string 2-5 - tied to another identical chord renders differently each time I try it. That is, sometimes I get 3 ties up and 1 down, sometimes 3 down and 1 up, and sometimes 2 & 2 - and when it renders this way, he ties cross, which is not good.

I'm submitting this now so I don't forget, but I will look at it soon to see if I can come up with something more concrete. I don't know how the algorithm for tab is *supposed* to work, so unless there is something obvious I see, I'll leave it to someone else to fix.