Since iOS 11.3.1 was released, updates are not caching to the MacOS server app on 10.12.6 or on 10.13.4 is the caching service.
This appears to be due to the url that the updates has changed domains from
"appldnld.apple.com" to "updates-http.cdn-apple.com" and this is not whitelisted in the Server app.
example from log
2018-04-26 17:23:07.431 #SudsCaLQ8/oL Request by "itunesstored/1.0" for http://192.168.74.155:49468/2018/ios/091-78097-20180419-1837C44A-4063-11E8-A17C-2CE0FBE594B0/com_apple_MobileAsset_SoftwareUpdate/348080d1ecd8fcb42ee21a48be506d66248db38a.zip?source=updates-http.cdn-apple.com denied because the source is not whitelisted
2018-04-26 17:23:07.431 #SudsCaLQ8/oL Failed with status code 400, reason code 66
I have logged this with Enterprise Support case:100519636930 who have been able to replicate the issue also.