I am a flutist and I will be playing flute in a harp ensemble. Is there anything I should know concerning tuning issues with harps while playing. Are there certain notes or octaves that are prone to being flatter or sharper than others?
The number of tuning issues depends on your skill level and intonation. Between the two of you, it is much more likely that you'll be the one out of tune. Harps are chordophones and the by nature of the instrument take forever to tune (much like a piano) and go out of tune with generally the same frequency as a piano if not properly maintained.
While their notes are simply plucked into existence, yours must be created by air which automatically increases variability in your intonation.
So if you want to avoid tuning problems with harps (or any other instrument for that matter) practice long tones and even breath management.
Allow me to recommend you touch base with your harp ensemble and ask them if there's anything you should know about what tuning system they will be using. I'm both a woodwindist and a harper, and sometimes harpers forget that woodwinds can't do everything, tuning-wise, they can.
You should be advised that harps, because of their variety of historical forms and because they are tuned manually by the player, all the time, can be and often are played in all sorts of alternative temperaments.
For instance, I play an late renaissance single-row 19 string harp for predominantly 16th century repertoire, so I have* my baby in an E-flat meantone tuning that is historically appropriate for my repertoire. One of many such options; the 16th century saw a profusion of temperaments. At least my A=440; other early music ensembles (especially those focusing on the Baroque period) tune to A=415 or A=460 or A=who knows what.
Many folk/celtic harpers use pythagorean tuning; you can tell them because they tune from a single pitch by moving in fourths and fifths. Others don't know for temperaments, and just use an electronic guitar tuner (or I guess these days there's an app for that and they use their phones) and wind up in the same A=440 equal temperament we all know and love.
I have no idea what orchestral harp players do. I'm guessing A=440 equal temperament, because orchestral.
In any event, you're going to have to ask your actually harpists/harpers what they're doing for tuning or temperament. Odds are, nothing odd is happening. But the fun thing about harps is that the options are much wider than for most (all?) other instruments. So maybe just touch base with the ensemble.
As to ranges being sharper or flatter, it can depend on the instrument, but generally when harps drift, they drift flat. That's because the very tension that holds them in tune can slowly pull them out of tune, such that the tension slackens. Slack tension, lower pitch. Also, the higher the tension, the more refractory the string; that can mean a tendency for higher registers to go first. But on a harp, it's not as simple as high pitch == high tension, because weight differences of strings.
But really, the thing on which likelihood of tuning being off most depends on is newness of the string. So you wind up with really idiosyncratic tuning issues. For instance, the string I most recently replaced on my harp is the A above middle C. That string is the first one to go flat; I'm constantly fighting with it, cranking it back into position. The strings around it are plenty stable.
* And by "have" I of course mean "keep putting back into, over and over and over."