It is important, as you say, that the Acaryas, at least as far back as BVT, do use the term ahaituki krpa, “causeless mercy.”  This fact led me to dig a little deeper into the term ‘ahaituki’, which as we know is an an adjective meaning having no hetu or cause.

The word hetu often means ‘motive’, ie a mental cause, especially in the sense of selfish motive. We find this use of the word ‘hetu’ in the Gita 2.46, where Krishna says, literally, “Don’t be a person whose ‘hetu’, ‘motive’ is action’s fruit. Ma karma-phala-hetur bhur. The Gita again uses haituka to refer to ‘motive’ at 16.8: the Asuras see the world as ‘kama-haituka’, i.e. everyone here acts with a selfish motive.

The Bhagavatam and CC mainly use the word a-haituki in the sense of ‘having no material motive’, especially in relation to bhakti. Thus we find the term ‘ahaituki bhakti’ at SBh 1.2.6, 3.29.12, 10.23.26, and in CC Adi 4.206, Madhya 19.172, Madhya 24.29, Madhya 24.146, Madhya 24.163,167, and Antya 20.29 which quotes Mahaprabhu’s Siksastaka — bhavatad bhaktir ahaituki tvayi.

Also, SBh 5.18.9 gives us ‘ahaituki mati’, ‘causeless/unmotivated thinking of Krsna.’

Then we have the interesting statement in SBh 9.25.22 that Dhruva went to an ahaituka-loka’ which Prabhupada translates as ‘a planet devoid of dry speculation.’ Prabhupada here takes hetu as ‘cause’ in the sense of reasoning on causality, as in the Gita 13.5, where Krsna speaks of the brahma-sutras as ‘hetu-mat’, ‘possessing ‘reasons’, i.e. analysis of causes.

Similarly and negatively, in Bg 18.22, Krsna describes knowledge in ignorance, tamasa-jñana, as ahaitukam, being irrational, or having no good reason.

My conclusion: by introducing the term ahaituki krpa, ‘causeless mercy’, BVT and other Acaryas are completing, and making explicit, a theologic circle implicit in Shastra, especially in the Bhagavatam 1.2.6-7. Here is my reasoning:

1. SBh 1.2.6 states that paro dharma, the highest dharma, is that which leads to ahaituki bhakti, devotion with no material motive.

2. The next verse, SBh 1.2.7, states that such bhakti-yoga quickly produces vairagya and jñana which are also ahaituka.

3. Krsna teaches in the Gita that He gives knowledge. Thus with no material motive, Krsna mercifully awards us spiritual understanding which leads to real detachment, vairagya, as explained in the Gita 2.59.

Conclusion: When a devotee offers pure ahaituki bhakti to Krsna, Krsna, who is the Supreme Pure, [Bg 9.7, 10.12] then reciprocates with spiritual blessings, which also have no material motive. Thus Krsna bestows causeless mercy.

Thus ‘causeless mercy’ does not indicate that Krsna treats souls unfairly, rewarding one and punish another for no reason. As we know, He explicit rejects this notion in the Gita. The Bhagavatam also emphatically rejects this idea of divine partiality, often in the context of battles between the Devas and Asuras. Krsna often intervenes on the side of the Devas, and the Bhagavatam repeatedly reminds us not to misunderstand such divine interventions as a sign of partiality in God. Krsna always has good reasons for what He does.