Filters with custom attributes not working properly

[Webguy]

We are using Phoca 6.0 but still an old issue is present. Perhaps we've set something up wrong or are missing a certain setting but I've run out of idea's.

Example
When selecting checkboxes in the Attribute filter "Kleur", in some situations the checkboxes disappear. It seems to happen with certain colors.

When navigating to the resulting url you can see the same issue.
https://www.fleev.nl/webshop/items?a[kleur]=blauw,geel
https://www.fleev.nl/webshop/items?a[kleur]=grijsblauw

Video;
https://share.zight.com/wbu40Oeb

Is there a logical, clear issue/solution here or should I provide some login details?

Best regards,
Peter