Client cipher always wins, even with Cipher server preference option set

Interesting, I couldn't find a bug fix article for your version though, but i know that beginning in v15, this feature is being taken out completely, so that it honors servers cipher suite preference.

https://support.f5.com/csp/article/K12390

I think you may need an engineering hotfix for your current version or go with an upgraded version.

Try to open a support case to get confirmation from F5 Support partners.