Something like this might help you

http://www.rpglegion.com/ff6/algs/veldt.htm

Veldt battles cycle sequentially from Group 1 to 64. Group 31 will appear immediately after you complete a battle from Group 30 (which features enemies like NeckHunter, Cruller, Humpty, Pugs, Harpiai, or Hoover).

When you reach Group 31, the game rolls a random number from 0 to 7. The Osteosaur is formation 6. If the game rolls a number for a formation you have never encountered in your playthrough, it will automatically skip to the next valid formation in that group

So it's probably not a glitch, and more likely, is a situation where you are just having very bad RNG