Yes that is the intended behavior.
Reeler's mini slot requires all three colors to match to win, that comes out to ~11% chance per spin. The rewards are usually common materials like Quartz, Coal and Copper. Titanium is in the loot table but only at about a 9% chance. That's roughly a 1 in 100 chance per spin to get Titanium from the slot.
In practice, that means a player would need to go back to the surface ~50-100 times to see Titanium even once. Personally I feel that is reasonable, since that same amount of time could be spent digging normally for Titanium, which is generally faster and more reliable.
I did consider cases like macro or afk farming, but since this is a single player casual game, I am not too concerned. If someone really wants to go through that effort to cheese it, I am okay letting them play that way.