• enable product build $g_enable_product_build = ON;
• create issue I1 without entering a value for product build
• create issue I2 and enter value B1 for product build
• goto View Issues page
• click on Product build
• get list with values [any], [none], B1 and an additional wrong empty value
• select the empty value
• click Apply filter
• filter field changes to [any] instead of keeping the empty value
• wrong list of issues is displayed, just I1 instead of I1 and I2

The wrong filter can be stored.
When loading again, you will see [any] in field Product build, think it is not filtered, but will not get all issues.