In the 3rd edition of the game, the Dark Angels have a few special rules to reflect their differences with more vanilla Codex Astartes-compliant Space Marine Chapters. One of them are the presence of the Ravenwing and the Deathwing as special units, and special rules about the Fallen.
Another special rule for the Dark Angels is intractable:
Dark Angels are very tenacious, but sometimes this will overcome their better judgment and they will stubbornly refuse to move when it might be in their better interests to do so. To represent this, roll a D6 for each Dark Angels character, unit or vehicle at the start of your turn if there is an enemy model within 24", in sight and within range of at least one weapon of the unit. On a roll of 2-6, the unit can move normally but on a roll of 1 the Dark Angels will refuse to move and must remain stationary. Dark Angels units that refuse to move may not move during the movement phase or the assault phase, but can shoot as normal. In addition, they count as being stubborn for the remainder of that turn and all of the following enemy turn, even if they are not usually classed as stubborn.
Important exception: This rule does not apply to Ravenwing units and characters.
(I don't link to the Codex for the 3rd edition, as I believe it is illegal to share scans online)
Note: units with the stubborn rule pass automatically any Morale checks.
When reading the Horus Heresy book series, I couldn't find any element that supports the claim that Dark Angels are tenacious, to the point that it sometimes overcomes their judgement. I would rather associate stubbornness with Rogal Dorn or Perturabo than Lion'El Jonson.
Is there any element in the lore of the game that supports the fact that Dark Angels are tenacious and stubborn, and that their tenacity can be a strategical problem for them at times?
"Tenacious" should be understood here in comparison to other Space Marines. Any background story is acceptable for the answer, be it from the time of the Great Crusade/Horus Heresy, or the "contemporary" 40k era, or anything in between. Old stories are acceptable too (my guess is that this part of their personality has been removed from the modern version of the lore - see my remark on the gameplay below).
I guess that the Dark Angels have been described as stubborn for out-of-universe and gameplay reasons. In the 2nd edition, Dark Angels and Blood Angels were sharing the same Codex Angels of Death (I didn't read it) and Dark Angels got their own Codex for the 3rd edition of the game. The Blood Angels had the opposite rule in their Codex: on a roll of 1 at the beginning of the turn, a Blood Angel unit would succumb to the Red Thirst and move toward an enemy to engage them. This (together with the Black Company and later an assault-oriented Dreadnought) made the Blood Angels a more offensive Chapter than the regular Space Marines Chapters. The stubborn rule may have been created to turn the Dark Angels into a more long-range-oriented Chapter.
