Here's a screenshot. There IS actually a white border around the text, but it's very very thin. Notice the "101" text in the lower right corner? Even at this larger-than-tablet magnification, it's very hard to tell there's any separator.
Thanks, I thought you had examples without white border and there would be a bug. As said above, the border is not scaled 150%, so it is too small for letters at his size, but OK for what is was meant.
Also, Locus beta 3.40.2.1 here is going nuts, drawing and redrawing the labels at two wildly different sizes. Here I've caught a screenshot with both sizes shown at once. Panning seems to trigger some re-rendering, and I had to pan a lot to line up things for the screenshot.
I can't reproduce this, but the really small ones are either wrong or, like in the screenshot with "4m" scale, are too much zoomed in. Until it's optimized for MF 0.12, street names are mainly optimized for zoom 16/17, where there was a chance to see them in previous versions. As street width scales with zoom, but the text size doesn't, the street names look tiny at ZL 22 in your example, but are actually just the same size as in ZL17. I don't think there's any sense in adding lots of code for optimizing for zoom levels that don't have a practical value ;-)
I have to disappear shortly from the internet for a few days, sorry.
Good for you :-)