While a bit delayed, I came across a similar situation and wanted to share my solution. I am on Acumatica 2023 R2.

I have been able to accomplish your expected functionality by using the “Contains” condition instead of “Is In”. The syntax for the list of values is only comma delimited, not comma-space, and all one string. You can also use a combo box parameter (I have mine called eBrand] in below picture) if you wish to dynamically select which values are to match.

I have found all of these to be equivalent for returning results that have a value of ValueA OR ValueB OR ValueC

Note that if a field can have multiple values, then the condition will match if any of the values are present. To elaborate, say we have the below three items with the multi-value attribute of Brand:
SKU             Brand                 
ItemA            ValueA
ItemB            ValueB
ItemAB          ValueA,ValueB

With the condition of “InventoryItem.BRAND_Attributes  Contains  ValueA”, the result will be ItemA and ItemAB.
With the condition of “InventoryItem.BRAND_Attributes  Contains  ValueA,ValueB”, the result will be ItemA and ItemB and ItemAB.

Thanks @stephenbologna39 , I have been using the OR conditions in my GIs to get around this. However had hoped there was a better way, there are many use cases for the ‘Is In’ condition, I would hope that one day Acumatica can inform us of the correct way to use it.

Hi @MarkOM 

I’ve also tried using ‘Is In’ like that on a few occasions, but have never gotten it to work either.  The only documentation I’ve ever found on it comes from the Generic Inquiry’s built-in help page.  This following comes from the help page under Conditions -> Value 1:

From what I can tell, it works with workgroups.  I don’t know if it works for other scenarios.

Generally whenever I need to match a field against more than one possible values, I have just used multiple condition lines connected with ‘Or’.

For example:

Hi @MarkOM  If possible, can you please share the GI, which you created?