It is not so much the copy paste, it is the value used for All, this needs to be "All" not "*" (similarly with the initialValue) <form version="1.1" theme="light">
<label>All handling</label>
<search id="base_search">
<query>| makeresults format=csv data="categories
A
B
C"
| table categories</query>
<earliest>-24h@h</earliest>
<latest>now</latest>
<sampleRatio>1</sampleRatio>
</search>
<fieldset submitButton="false">
<input type="multiselect" token="categories">
<label>Categories</label>
<choice value="All">All</choice>
<default>All</default>
<initialValue>All</initialValue>
<fieldForLabel>categories</fieldForLabel>
<fieldForValue>categories</fieldForValue>
<search base="base_search">
<query> |stats count by categories </query>
</search>
<valuePrefix>testCategories="</valuePrefix>
<valueSuffix>"</valueSuffix>
<delimiter> AND </delimiter>
<change>
<eval token="form.categories">case(mvcount('form.categories')=0,"All",mvcount('form.categories')>1 AND mvfind('form.categories',"All")>0,"All",mvcount('form.categories')>1 AND mvfind('form.categories',"All")=0,mvfilter('form.categories'!="All"),1==1,'form.categories')</eval>
<eval token="categories_choice">if('form.categories'=="All","categories=\"*\"",'categories')</eval>
</change>
</input>
</fieldset>
<row>
<panel>
<html>
Categories: $categories_choice$
</html>
</panel>
</row>
</form>
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