There is a bug in SAP UI5 1.71 which causes the Action Sheet to open in the wrong position on mobile screens. The image below is from the issue logged against the openUI5 project on github.
Issue Link: https://github.com/SAP/openui5/issues/2804
The fix is shown in the resolution link.
The code shown (at the resolution link) is compiled into multiple CSS files which are part of the UI5 bundle provided by Neptune for the mobile app. To fix the issue you need to find instances of the code for the CSS class selectors and delete the offending lines. There will be several instances across several files.
If you have difficulty resolving the issue get in contact with us via a support ticket.