Hi anonymous user
The issue is really strange, do you mind create a new simple report and use the lookupset function again for test.
If this function still doesn't work in v150.1404.0 but work in v150.900.148,you may post the issue at (feedback forum):
Once you get the official solution or feedback , you could attach the reply in the thread .
Thanks for your support and understanding.
Here is a release notes for Report Viewer controls for your reference.
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