I can clearly reproduce when the reported issue takes place. It like comes from the two path elements between the two text elements containing main body. I suspect a wild pointer problem in the matrix attribute.

This bug at gnome shows also an issue with a matrix element: https://bugzilla.gnome.org/show_bug.cgi?id=634324

The result to text elements is equal to multiplying font-size with scaling of previous path elements which leads to font hinting and upscaling text again to the intended font-size.

I've analyzed the code and circled the error in a function from rsvg-cairo-draw.c of librsvg

rsvg_cairo_render_path

By commenting out condition line at the beginning and end of this function for layer stack it fixes the problem.

if (need_tmpbuf)
    rsvg_cairo_push_discrete_layer (ctx);

Excluding this push-pop operation of context makes things like scaling permanent for path rendering.

Difficult to say if removing the condition has unforeseen consequences. I'll try it next week.

Fix should be included in librsvg 2.40.13.

I think that extreme scaling in either direction will cause problems. At least that's what I observed when this SVG got rendered like this. The setup here is that for mathematical reasons, the circles there have radius 1, so there is a scale factor of over 100 applied to the whole image via its viewBox. As a consequence, the vertical positioning in rsvg is too low, and within a span of text there apparently is no horizontal advancment between glyphs, so they all buch at the same position. Both of this could be either due to font metric getting rounded inappropriately, or due to some transformation not getting applied to said metrics. I managed to work around the issues in later versions by manually scaling the text part, but that's rather annoying. I have librsvg 2.40.13 installed locally, and at least my issue is not solved by that version.

(Some affected files: https://commons.wikimedia.org/w/index.php?search=insource%3Aphab%3AT65703)

That bug is partly fixed on the new jessie image scaler using 2.40.16 (tested locally, it's not yet pooled into the set of active scalers in production).
I have scaled "Hyperbolic ideal triangle and its incircle.svg" on the new and the present image scalers and put the results at https://people.wikimedia.org/~jmm/svg/ :

hyper-old.png is the result with the current scalers. hyper-new.png now has correct positions of the A,B,C,D points, but the "|AB| ≈ 0.55" etc. is still distorted. That's probably a separate bug to be reported upstream since 2.40.16 is the latest upstream release.

In T65703#2401510, @MoritzMuehlenhoff wrote:

hyper-old.png is the result with the current scalers. hyper-new.png now has correct positions of the A,B,C,D points, but the "|AB| ≈ 0.55" etc. is still distorted. That's probably a separate bug to be reported upstream since 2.40.16 is the latest upstream release.

That is clear bug T36947 because font-size="0.12px" is used.

Seems to still be an issue even though the image scalers have been updated to 2.40.16.

Compare:
https://upload.wikimedia.org/wikipedia/commons/thumb/9/96/Ikaros_solar_sail_key.svg/203px-Ikaros_solar_sail_key.svg.png
https://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Ikaros_solar_sail_key_plain.svg/203px-Ikaros_solar_sail_key_plain.svg.png
In theory, those should both render the same.

Please be patient and see T112421 in the meantime.