Please could you let us know the LilyPond version on which you observed
the problem?
I'm using version 2.15.39, but I've been noticing this behavior since 2.14.2 and perhaps even older versions, those 2 are the ones I can confirm.
Also, could you tell us how you envisage using this feature. As a
player, I find the reminder on 8va passages useful at line breaks. I'm
asking the question so I can determine whether this would be a
real-life, useful facility or simply whether you have found a
rarely-used corner case.
- First of all, in order to make some transcriptions/arrangements of a work that does have an 8va/15ma/etc. passage it would be good to produce it similar on LilyPond, sometimes those 8va passages do not repeat the "8va" text at line breaks on paper published scores you buy.
- Another one is to make it comfortable to Sibelius users or so to have an 8va line in a similar way they are used to (afaik Sibelius don't repeat the "8va" text). I know LilyPond doesn't need to imitate behavior from other software (LilyPond is already cool anyways), but users like to have options (see below).
- Last but not least, it would be good to have options available, whether or not you need the duplication of "8va" text the user would really appreciate being able to decide how he/she wants the output to suit his/her needs.
Hope that helps!