Figured bass switches direction!

[Gilles Sadowski]

Hello.

In the following code, the figure direction switches: In the second bar
the flat is printed above the 6, and the "6 5" is printed "5 6"!

%---
\version "2.8.7"

<<
  \relative c' {
    \time 3/4
    \clef F
    \key c \major
    
    a4 e f |
    f4 c d |
  }

  \new FiguredBass {
    \figuremode {
      <6>4 <6 5/> <_-> |
      <6 _->4 <6 5/> <5> |
    }
  }
>>
%---

I guess that LilyPond is aligning identical numbers but this is rather
confusing.  Is there a way so that the numbers are always printed in input
order?

Thanks.
Gilles