First of all: I'm assuming you made sure the neck was perfectly straight when you leveled the frets, preferrably using a notched straight edge. If not, bad things can certainly happen. I'm also assuming the fret buzz appeared AFTER you had leveled the frets, because if it was also present BEFORE you leveled the frets... Well, that would strongly suggest that the problem is caused by something else, right?
Anyway, you're saying the buzzing "seems to" come from higher frets. Does that mean you're not sure where the buzzing is coming from? If the buzzing is only present when playing higher frets then yeah, the buzzing must obviously come from higher frets.
But if fret buzz is present when you play lower down the neck as well, it would strongly indicate that you have too much neck relief, rather than high frets higher up the neck. Especially if the buzzing is happening in the middle of the neck, say somewhere around 8th-12th frets. This is the part of the neck that will be at the bottom of a "ramp" if you have too much neck relief.
A neck made from poor quality, soft wood can also cause issues like this. Some basses just can't handle a correct amount of neck relief or low action without buzzing, no matter how perfectly you level the frets. The quality of the wood matters, a LOT.
Buzzing lower down the neck, like below the 5th fret, would of course indicate insufficient neck relief. Setting up a bass can be a balancing act.
You also don't mention how low you set your action, or how low your nut slots are filed. You should be aware that if the nut slots are too high, and you adjust your action "by numbers" rather than by experience, you will end up adjusting your saddles too low, which will definitely cause fret buzz. If you file the nut slots lower you should be able to raise the saddles, get rid of buzzing and still end up with the same (or lower) action at the 12th fret. Filing the nut slots to perfection is something I do NOT recommend doing "by numbers" using feeler gauges. This really has to be done by "feel" as every instrument is different. The choice of strings and playing style will also matter. Filing perfect nut slots is a skill that requires some experience. It is by far the most difficult thing to get right when servicing a guitar. It is much harder than leveling/crowning the frets. When in doubt, leave the nut slots slightly on the high side. They may not be perfect, but probably still considerably better than the factory nut slot heights. If you file the nut slots too low... well, there are a couple of tricks you can use to reverse it (sort of...), but none of them are ideal IMO.
Pickups too close to the strings can also cause buzzing.
Of course there may be a problem with your fret leveling procedure as well, but from your description I strongly suspect a setup issue. Especially if this is happening to more than one bass.
PLEK... Personally I don't recommend it. The level of accuracy may seem impressive, but do you really NEED that kind of accuracy? Play the bass for a couple of weeks and you've already worn the frets past the level of PLEK accuracy. Traditional fret leveling, when done properly, is just as good IMO. In my opinion PLEK treatment is comparable to using tuners with 0.1 cent accuracy to adjust the intonation, and then realize that when strings have been played for a week the intonation will have shifted to the point where that super-accurate tuner will tell you the intonation is off (while it is in fact perfectly fine).
