When using subtractfeeamount=true with the sendtoaddress RPC call, the wallet does not always respect paytxfee values and seems to occasionally employ an arbitrary (and very high! 0.004 BTC/KB at latest glance) fee. Strangely, after sending a standard call with subtractfeeamount=false, subsequent calls that include subtractfeeamount=true seem to respect the value set by paytxfee.