I also recently tried the Polar Brown in my Dollar 717i demonstrator to use for underlining & margin notes in books. The flow in this pen wasn't great & including some skipping even after thinning a bit with distilled water... The Polar Brown, however, does work great in my old Parker 88.
I have the blue $ Demonstrator pen and use nothing but Polar inks in it now, having no problems like you've described.
I find that my Polar Brown settles a bit in the bottle, a white outline/ring seen when inverted [no idea yet what the white substance is but it's in both of the two bottles I have].
If I turn the bottles over, use a swirling motion to remix [splashing/swirling the ink against the bottom], the white [whatever it is] reconstitutes into the ink and works perfectly for me in all pens I've tried it in including the $ pen.
Normally I keep pure Polar Blue in my blue $ Demonstrator, maybe just a "blue" thing but I did try the Polar Brown in it and had no issues like yours.
I did notice that the Polar Brown lubricates so well that a tiny amount [like a drop] gets past the plunger seal and settles behind the gasket, whereas my non-Polar inks rarely do.
It really lubricates the piston-fillers nicely, including my piston converters.
I love the Polar inks and have all 3 colors now [Blue was my first, then the Black and the Brown]; I only wish I could afford to buy each one by the caseload.
Oh yeah, I too get some
nib creep on certain pen nibs when using Polar inks, usually the nibs with decorative etching on the top surface are the worst but still not too creepy.
That doesn't bother me, since the ink works so well in general and is the slickest, most permanent stuff I've ever tried [Polar Blue seems to be the slickest out of the 3, I have no idea why].
As permanent as Polar inks are, they seem to wash off my fingers easily with a drop of Dawn dish washing liquid and a little scrubbing with a sponge.
A little tougher than some inks to flush from my pens and converters but it does fully rinse out when needed and has not yet stained any of my converters or see-through pen barrels with use.