Licenses can be complicated, but I will try to explain. Basically this case is like trying to put a large ball into a small basket.
Note there are several CC licenses with different requirements and in different versions. They can be seen on https://creativecommons.org/licenses/ .
Take Creative Commons Attribution-ShareAlike 4.0 International License (CC BY-SA 4.0 for short), which is the one Mapillary shares the images as.
As a license it tells you what you can do with the images and how you can share them with others, if you can share it. So the license applies if you give the image away, if you alter the image or use the image in a larger work.
The Attribution (BY) part means, that you must always give credit to everybody who contributed to the image - that is e.g. the user name of the Mapillary user.
ShareAlike (SA) is special for Open Source and Free Software. It states, that you if you share the image, you must give others the same rights to the image as you got. So you cannot take a ShareAlike image and tell others they cannot copy it, because you have the right to copy it.
If you read the terms and conditions of Mapillary, section 1.3 "Mapillary User Licenses to Us." it specifically states, that they can pretty much do anything. They are not obligated in any way to give any attribution.
In CC-BY the BY is a shorthand for Attribution, so it requires you to tell who made the original image. But when contributing to Mapillary you allow them NOT to do this. That means, you cannot publish CC-BY material on Mapillary because Mapillary demands there are no such restrictions on the images.
If you publish it on Mapillary, you are voilating the CC-BY license and if Mapillary accepts it, they are essentially publishing pirated images.
The whole thing is extra confusing because Mapillary is a very open company, but actually only accepts images under a very closed license. Then afterwards they use those rights to share the images under a CC license, but they are not required to do this and is allowed to alter this at any time.