Check your dataset for null values. If it contains a null value for anchor event, but your report parameter is set to not allow nulls, RS will revert to not...

Can you insert the sscreenshot of report parameters properties?

I would suggest you to first try recreating the report as a new one.

Also, what I understand is that the values are being populated in the dropdown and just...

I once had such issue when I was using multiple multi valued paramaters. I restarted Visual Studio and then recrerated the report (copy paste the report, create dataset, refresh fields...

That's weird. Does this happen when you preview the report or when you deploy the report?

As Wayne said, populate the default values from query (using the same dataset that you used to populate the Available values) and values field same as the Available values -...