We also encountered this today. Published three layers to AGOL as a service, point symbols did not show up at all in iOS Collector, but data and layers were there in the web map. Poly layer was fine.
We published 'dumb symbols' (default point circles with color), from ArcMap and they displayed from iOS Collector/AGOL with no problems
The hosted features were published from Windows 7 and 10 with latest updates.
Does not appear to be an issue with our Portal. We pushed our fancy symbols from ArcMap to Portal and found an offline configured basemap (NatGeo World, no stupid esri credentials asked for...) to use for our remote field crew
Hopefully esri has this resolved by conference time... cuz we LOVE to complain!
ArcMap 10.5.1
Portal 10.6.1
New Collector for IPad