This appears to be buggy. For the time being, if your actual code is similar to what you have posted you can work around the problem with something similar to the following:
Just wrap your image tags with a div specifying the max width, then specify the image width as 100%. There are other, similar variations that would also work.
Note: this post would be more appropriate in the
dompdf support group. The purpose of this group is to discuss the internals of dompdf and future development. While your issue points to a problem in the code, the overall topic is one of support. You could also
post a bug report if you like, since this appears to be due to a bug.