It probably is (but not 100% sure), because the few times we've tried bringing it up with the devs, we've heard nothing.
Based on what Mark said on how the quests work, the chance of getting a two-line quest is the percentage of fish types you've completed one quest for, times some other number (as it doesn't appear to be a 100% chance). The chance to go from 2 to 3 lines, etc. works in a similar manner.
The "undocumented feature" apparently takes the Dungeon fish into account in the equation a little early, but either way, at one point between 100 & 106, the number of fish in the equation increases by 12 (the number of dungeon fish types), making near 100% chances drop by 2/9 (X/42 versus X/54)
For trying to get a 6-line, that's an initial (.777)^5 blip, or an over 70% drop in your chance to get a 6-liner, if you'd already completed at least 3 quests of all of the original 42 (shallow/deep/crustacean), once it starts counting all 54.
It typically takes about 50-60 quests with at least one dungeon fish, to get back to where 4-6 line quests become common again, and probably over 100 such quests for 5 & 6 lines to become dominant again, just from the basic probabilities (and said probabilities probably mean the actual number of quests will be double this, with half of them having NO dungeon fish).
yielded my first 110 from Zann "the naked gargy guy" fish monger 
