It seems that the step attribute of the mobi:inputText has to rendered from a full page refresh as changes to the step attribute are not being applied to the DOM from a DOM update.

<?xml version='1.0' encoding='UTF-8'?>
<partial-response><changes><update id="inputPage:inputForm:numberInput"><![CDATA[<input autocapitalize="off" autocorrect="off" class="mobi-input-text ui-input-text " id="inputPage:inputForm:numberInput" name="inputPage:inputForm:numberInput" placeholder="Number input" step="1.2" style="" type="number" value="9.6" />]]></update><update id="javax.faces.ViewState"><![CDATA[-8413699931142451045:-4093515930129722581]]></update><eval><![CDATA[var iceFormIdList=['inputPage:inputForm']; ice.fixViewStates(iceFormIdList,'-8413699931142451045:-4093515930129722581');]]></eval></changes></partial-response>

DOM after update:

<input autocapitalize="off" autocorrect="off" class="mobi-input-text ui-input-text " id="inputPage:inputForm:numberInput" name="inputPage:inputForm:numberInput" placeholder="Number input" step="0.8" style="" type="number" value="6.4">