Has fuel boiling ever been an issue for high altitude aircraft?
Yes, e.g. U2


  Both planes, equipped with the older -37 engines, landed at Kirkland Air Force Base Albuquerque, New Mexico. After each development flight, a careful accounting is made of fuel consumption. A special fuel, dubbed lighter fluid, was developed by Shell Oil Company specifically for the Angel, and the finished product was shipped to Nevada in tank cars labeled LF 1A. This blend will not boil at the low pressures encountered at altitude, yet will still give adequate air starts. It is so involatile that fire seldom follows a mishap.
  
  A simple 100 gallon-slipper tank has been developed to fit each wing for extremely long flights. These pressurized tanks contain enough fuel to carry the Angel to cruising altitude where they have no significant effect on speed or range. Even after the addition of an external drag chute, three times the normal oxygen supply, improved breaking and an autopilot, the final all-up weight was within ten pounds of the original proposal.


https://www.cia.gov/library/video-center/video-transcripts/u2-developments.html