Solve a strange bug: combobox can only choose the first one

When writing pages, there are always various pits. When writing with easyui, I find that when selecting the combobox drop-down box, only the first one can always be selected, and when the second one is selected, it will jump to the first one.

code show as below:

<div style="margin-bottom:20px">
			<div>Software version:</div>
			<select  name="softwareVersion" class="easyui-combobox" data-options="panelHeight:'auto',valueField:'versionID',textField:'versionName',url:'baseVersion.do'" style="width:100%;height:30px;"></select>
		</div>

After checking for a long time, I found that the versionID of valueField:'versionID' is wrong, it should be versionId, and the letter case is wrong. Why does this problem occur?