Yes, you seem to be right. The CMS must have done a conversion to the image. The file I uploaded was 1.2MB. When I download it from the website, it is just 34kb. So, somewhere in between the data has been removed / lost. I will investigate this but many thanks for your "outside view".

Be glad the CMS converted the image. 1.2MB is way to big to use on a website. Imagine you have 3 or 4 of those images on a page. It would take ages to load on slower connections, not to mention the impact on data limits some providers have.
As you can see the quality of the 34kB image as displayed is fine, there's really no need for a huge image of 1.2MB. If you want users to be able to save the image in the original svg format make it a separate download.

To solve your problem create a much smaller png from the svg image and use that on the site.