How is the flare manifesting itself? Are you seeing artefacts and reflections, or an overall loss of contrast (veiling flare)? As others have said, it would help us to help you if you could provide some example shots
I use this lens quite frequently on my K-3 and K-3II. I wouldn't say it's an outstanding performer in terms of flare, but nor is it especially bad. About average. I always shoot mine with the original hood fitted, and most of the time without any filter attached. Depending on weather conditions, I occasionally fit a Hoya HMC Slim UV filter to protect the front element (from spray, sand etc.). I've had relatively few problems with flare, and nothing that a carefully-positioned hand couldn't resolve.
Tiffen's UV filters have a very poor reputation for flare. I don't know if their CPL filters are any better, but it's reasonable to assume they'll be just as bad. I think this is the most likely cause...
Have you done tests with and without hood, with and without filter, etc.?