I can't say I understand at all why this would happen, but this was reported here (https://groups.google.com/g/xnat_discussion/c/NI_83xNnFtw), and I can recreate it as well. It really does seem to be a hard cutoff at 10 [unique SCP receiver ports] where having 9 receivers works fine, but adding the 10th causes the compressed uploader to stop working entirely. Moreover, using a port already in use for the 10th doesn't cause the issue, and deleting the 10th unique port's receiver causes the compressed uploader to immediately start working again. Unfortunately, there doesn't seem to be anything in the XNAT/tomcat logs.
It also seems to be specific to the new implementation of the compressed uploader as I was able to have 10+ unique port receivers in 1.7.6 without seeing this issue.