The HP FB's are fairly unique to that problem by design. Fortunately the 1625/16H build are not configured remotely the same. Not to sound like a broken record lol, but a few pictures would tell us a lot more. White is tricky in this case due to the nature of the ink and the practical use of it by most shops. It may be in date, but how old is it? That line of white ink works very well when it's young, but it separates and ages terribly. You really have to use that stuff for it to behave at it's best. You said the trail comes after text, so it sounds like you're printing text in white or backing with it? Unless it's LARGE text, then you're asking white ink technology to perform in it's better categories - your'e not asking for junk work.
Before moving vacuum pressure out of spec or looking for machine/head issues, here would be my troubleshooting steps I most commonly find as the culprit in shops with various equipment:
1. Most people always run an unlinearized white output at 100%, this is a common mistake. White has several issues at physical 100% output - droplet size mostly, then curing, adhesion, interlinking, cracking, etc. White reflectance opacity is achieved on most printers at 15-20%. Transmission opacity starts at 50%. If you need 100% dump opacity, then 2 hits of 50% will look better than 1 hit of 100%. (So run 3 layers, instead of 2). If you calibrated for linearized white (which I wouldn't recommend unless you know you need it), then none of that applies to your application.
-----Solution Test: Run Color over 50% white, see if the problem remains. Then Color over 75% white.
-----Try Grayscale just for kicks. White opacity drops dramatically by nature. Usually a much better laydown. May not be as opaque as you require depending on your application.
2. If the ink is older than 3 months, swap it out with a NEW bottle (not 6 months old but unopen!).
-----Solution Test: With new ink, do 3 full secondary tank dumps then purge/wipe and start testing. If the ink is older than 6 months, it is absolutely in the "misbehave" category in my book. Yes it will work technically, but over 6 months old in that configuration and many of those printers cannot lay it down clean for fine text.
Start there and let me know, I'll be happy to help further. Running a more appropriate droplet size and pass/output amount on that ink will do wonders, and newer ink on top of it. White is a technology that OEM's have not done a great job of educating us about. Even if you get it looking good off the press, if you dump 100% pass backers for decals and stick them out in the sun, they will often crack in just a few months on an inelastic substrate. And remember that if your end product ends up on an opaque backer of any color, you never need 100% white, just enough to provide a whitepoint while the underlying object creates opacity.