For some reason a lookup field in a Lightning Web Component is not working properly when accessed from a mobile browser. Not working properly means that the autocomplete function does not work properly. This works fine when accessing the community page on a desktop, but not a mobile.
This is a Lightning Web Component with the field below being a lookup to the Contact object within a lightning-record-edit-form compenent
<lightning-input-field field-name="Related_Contact__c"></lightning-input-field>
Best Answer
Apparently lookup type fields are not supported by this component in Mobile...
https://developer.salesforce.com/docs/component-library/bundle/lightning-input-field/documentation
The solution is custom component as per the example: https://github.com/pozil/sfdc-ui-lookup-lwc