The same happens to me.
With sinking BTC-prices the 0,0035$ per 10GH maintenance fee ( which is calculated in BTC) rises.
My problem is however why the payout (from which the fee is to be subtracted) declines over time.

In only 12 days my daily average lowered (from first 4 payouts) by about 15%
And the daily payouts are declining daily. As for now, I reached 88% of the daily average.
If this trend goes on, by the end of contract I could have easily put the money in mainstream coins and let it sit there til mooning or regular rise.

The problem is that after one day I made an additional purchase which is stil pending.
The payment in BTC came through after several hours and since then it sits there unspent.
I wrote a ticket much too late (4 days after) since I believed with solving their web issues my will be solved too. Now I reached day 10 of waiting for their reply.
All this hassle. I should have put my bet on Genesis Mining. I didn't since I don't trust altcoins to rise from ashes like Bitcoin did so many times.

Yes, every reinvestment is a contract of its own and will last about 365 days.
I do not know this by experience, I read about somewhere else plus. Also I applied logic. Since the initial purchase is limited to a year also every purchase per reinvest must be limited to one year.
Also there is this calculator I linked below.
https://whatsmyip.com/hashflare
You can set your initial investment and the hashrate you got.

I'm one of the people who tried first buying a little hashrate, then deciding several days later it is worth buying a decent amount of hashrate.
The second purchase is stil pending. I recon the 14 days you have to wait for withdraw, after registering or setting/changing wallet adress also counts for purchases.
It is nowhere clearly stated but many people who make their first purchase are getting through. And I am sitting here and presumably waiting for the 14 days period to end so my second purchase can start.
Wrote a ticket and the stated time that I have to be patient for an answer is after christmas eve.

Since I trust HF (do I have even a choice after making payment in BTC?) I planned to reinvest for about 4 to 6 months. Withdraving BTC to a trusted wallet when I can get initial investment plus 10%.
The rest I'd just let sit there for the cummulative contracts to end and then decide wether I should reinvest.