Solved on a re-upload of the same nominally cleaned version, so still puzzled.

Enforcing a check for 'invalid' SVG on upload would be useful

https://upload.wikimedia.org/wikipedia/commons/archive/9/96/20170405141458%21UK_traffic_signs_542.1_%281981%29.svg

being the glitched version,

Full specific links welcome which allow quick reproducing. I assume you refer to the first image version (full size) being cut at the corners in (some? all? one?) PNG thumbnail version, in contrast to the correct PNG thumbnail of the last version?

In T162263#3158213, @Aklapper wrote:

Full specific links welcome which allow quick reproducing. I assume you refer to the first image version (full size) being cut at the corners in (some? all? one?) PNG thumbnail version, in contrast to the correct PNG thumbnail of the last version?

Exactly

Looking at the diff between the most recent glitched version and the fixed version, the width and height definitions look like the potential culprit:

<    width="300.00003mm"
<    height="217.8736mm"
<    id="svg2"
16,17c9,11
<    inkscape:version="0.92.1 r15371"
<    sodipodi:docname="UK traffic_signs_542.1_(1981)_WIKIMEDIA.svg">
---
>    id="svg2"
>    height="3293.8369"
>    width="4535.4336">

rsvg is probably not rounding sizes in mm like inkscape. We're due to upgrade cairo/rsvg as part of T170810 and T170810, it'll be worth checking if the more recent versions behave better with that file.

In T162263#3590568, @Gilles wrote:

rsvg is probably not rounding sizes in mm like inkscape. We're due to upgrade cairo/rsvg as part of T170810 and T170810, it'll be worth checking if the more recent versions behave better with that file.

These updates have happened now so someone could re-test

Purged that file, the issue is still the same.

Issue still occurs with rsvg-convert version 2.48.4, so just waiting for an update won't work.