Not sure if this has been reported already, but here goes:
BUG: MapImageLayer does not show in mobile browsers on version 4.19
Reproduce: Open the following sample on a mobile phone (I've only tested Chrome and Samsung Internet browser on Android): https://developers.arcgis.com/javascript/latest/sample-code/layers-mapimagelayer-sublayers/ In my case the mapimagelayer does not show (you can only see the basemap). This bug is only present on the 4.19 version. 4.18 works fine. The bug is not only related to the sample and the service being shown there. It occurs for all mapImageLayers I've tested (several different services).