Hey Mark,
I have the same experience on both my Mistral and Polaris, although my Polaris seems a little worse about it. I have both now set a little on the low side to not FF at low pressure as at the tail end of a dive I'm generally not looking to burn through my reserve any quicker than I have to... Interestingly, I haven't noticed the issue with my JetAir, but I don't dive it near as often. I just write it off as the cost of 3 moving parts and 1 o-ring, remembering something maybe in Roberts' book, about this being a sport for those with "well developed lungs."
If I'm smart and thinking enough, which I'm generally not, I save my beautiful yellow-hose Mistral for the "pretty fish dives" or at least my second dive of the day when I've had a chance to see the lay of the land (water), conditions, and get comfortable. This was precisely my mistake on my first dive at SHF/SS this past March. New location, totally unknown conditions with unexpectedly heavy current (spring flow), wearing more rubber & lead than usual, excited to see friends, anxious to "show off" for boats & camera with Big Yellow... and I realized about 5 min. in I wasn't quite as "well developed" as I could have been in the lung department and could have used a bigger squirt of air... Swapped to RAM for 1st dive and everything beautiful, back to Mistral for later dives, everything still mostly beautiful.
Moral of the story: single-stage'res, not a great choice when multiple unknowns & variables at play, but wonderfully simple & dependable for those dives when you're just puttering around.
Fred