Clearly masturbation with lust is worse than masturbation without lust, but that doesn’t answer your question.
The Catechism does not appear to clearly to answer your question either. “ By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure” (emphasis added).
A sexual release is not the same as sexual pleasure. Therefore this action does not appear to violate the Catechism, perhaps avoiding the defined activity.
That doesn’t really answer your question either.
I suspect, regardless, that the action is still sinful.
It stimulates the sexual organs which can form or perpetuate addictions. It still risks scandal if discovered.
However, mostly it takes a sexual act out of the forum for which it was designed. Masturbation without lust still risks medical aspects like skin chaffing (and bleeding), being unable to perform within marriage (including a future marriage), and unknown cancer and brain chemistry risks. If God were to reveal to me that I was not able to conceive the child I was supposed to have done because I had masturbated in this faction, I cannot imagine my shame and sorrow and sense of lost. As we are called to keep our bodies as temples of the Holy Spirit, non-lust masturbation would still seem to be incompatible with this.
We have no right to expose our bodies to such risk (similar to the reasoning why any self-harm is bad), as the only potential justification, that it is arguably better than lustful masturbation, is already gravely intrinsically evil. Still, just as it is better to “only” beat someone up instead of murdering them, if one commits non-lustful masturbation as opposed to the lustful version, it is probably better. This does not make the act non-sinful, just as beating someone up is still sinful, even if you were considering a murder as an alternative. Non-lustful masturbation would likely still constitute grave matter, which if combined with full knowledge and full will, would make it mortally sinful. I suspect in practice it is usually not mortal since the will would seem compromised, but this presumes much, and should still be confessed and discussed with a priest.