The last note of a bar is a trill with a nachschlag. I can move the nachschlag back before the barline easily with a switch. But: if the bar in question is followed by a repeat sign, the grace-notes hop back into the following bar, where they don’t belong. This is a bug, it seems to me. I can drag them to where I need them in engrave mode, but that’s an ugly solution to the problem…
In the situation you have described, there is a widening of the gap between the grace-notes and the normal note but because the grace-notes stay where they are, the normal note is offset to the right.
Interestingly, if the barline is a start repeat barline ( ||: ), the nachshlag does allow itself to be placed before the barline when enabling the Grace note before barline switch in Properties.
Also, if the barline is a back-to-back repeat ( :||: ), when the Grace note before barline switch in Properties is enabled, the back-to-back repeat splits into separate end-repeat and start-repeat barlines and the nachshlag is placed in between them.
wconable, that is, because placing grace notes before a bar line is something different from a Nachschlag of a preceding trill. Most of the time we can use this ‘placing before the bar line’ as a workaround, we but especially in this case one should put the Nachschlag notes either into a different voice or write them as a tuplet.
I don’t mean to be rude, but I do of course understand what you’re saying about the difference between anticipatory grace notes that could be placed before a barline and a Nachschlag; I’ve been a professional musician for more than 60 years. The Nachschlag is the ending of the trill. Surely the situation I’m describing is common enough that it’s not unreasonable to suppose that Dorico should be able to handle it as gracefully as it handles thousands of other subtleties in our music notation. I suppose that it’s an unintended consequence of other choices that my very limited comprehension of all that Dorico is doing behind the scenes doesn’t allow me to see.
And if you use a simple double bar (||) the anomalous behavior doesn’t manifest itself either. Would like to hear from Daniel or one of his colleagues about this admittedly minor matter.
wconable, I always had the impression that the “placing before the barline” is an actual workaround we can use for a Nachschlag (which belongs to the preceding note)- until Dorico comes up with a final solution. In other words, we all keep our fingers crossed and are happy about the fact, that we can get quite far with the still temporary workaround. 
I remember a time, when still using Sibelius, I would write the Nachschlag as grace notes before the trill note and manually dragging them and placing them after the trill note, hoping everything would stay in place…
What’s happening here is that, as Steven points out, Dorico has some special rules for the placement of grace notes relative to repeat barlines. When you have an end repeat barline (:|), the barline itself is considered differently, and consequently the grace notes can’t be positioned before the barline; you see why when you then add a start repeat barline (|:) at the same position, because now the grace notes go before this barline.
Yes, I see. The issue of Nachschlägs remains.