Has a parable originally used by Jesus been left out of Luke 14? (Luke 14:7-11, RSVCE)
  Now he told a parable to those who were invited, when he marked how they chose the places of honor, saying to them,  “When you are invited by any one to a marriage feast, do not sit down in a place of honor, lest a more eminent man than you be invited by him;  and he who invited you both will come, and say to you, ‘Give place to this man,’ and then you will begin with shame to take the lowest place.  But when you are invited, go and sit in the lowest place, so that when your host comes he may say to you, ‘Friend, go up higher’; then you will be honored in the presence of all who sit at table with you.  For every one who exalts himself will be humbled, and he who humbles himself will be exalted.”


One finds here no parable or  story illustrating the importance of humility , but rather plain teaching.  In common parlance, a parable is a short allegorical story designed to illustrate or teach some truth, religious principle, or moral lesson, or a statement or comment that conveys a meaning indirectly by the use of comparison, analogy, or the like. That puts one in doubt as to whether a parable which Jesus in deed used, has somehow been taken away from Luke 14. 
My question is: have there any official studies, say, from the side of Catholic Church, undertaken to establish the possibility of such an omission ?
I think you might be loading a 21st century definition of the English word "parable" onto a 1st century Greek word.

παραβολή [parabole] is also translated "proverb", "figure", "saying", and "teaching" elsewhere in the New Testament, as well as in the Greek Septuagint translation of the Old Testament.

Examples:


  Luke 4:23 (RSVCE)
  
  
    And He said to them, “Doubtless you will quote to me this proverb [Gr. παραβολή], ‘Physician, heal yourself; what we have heard you did at
    Capernaum, do
    here also in your own country.’”
  
  
  Hebrews 9:8–9 (RSVCE)
  
  By this the Holy Spirit indicates that the way into the sanctuary is not yet opened as long as the outer tent is still standing (which is
  symbolic for [lit. a figure - Gr. παραβολή - of] the present age).
  
  Sirach 1:25 (RSVCE)
  
  
    In the treasuries of wisdom are wise sayings [Gr. παραβολή],   but
    godliness is an abomination to a sinner.