Every wholesome citta & cetasikas come with either upekkha or sukha vedanā as I understand.
How is it when one does kusala, lets say, sense restraint, and feels dukkha at the same time?
Is it because there is additional citta, dosa-mula citta present as a resistance to arisen sense-desire?
Is there something more present as well?
Any ideas?
In this case you mention there could be many scenarios.
Genuine sense restraint - i.e kusala citta- always comes with pleasant or neutral feeling as you say.
But there can easily be a kind of "sense restraint’ In the way of metaphorically gritting ones teeth and just enduring - so as you suggest dosa mula citta in that case.
Also cittas arise and pass away very quickly so there might be moments with dosa and moments with genuine detachment and then moments of desire and so one…In fact that is the way of daily life…
Also see this thread. Issues with meditation - strange experiences, unpleasant feelings: advice
As you know mere effort and good intentions alone are never going to work. It is the understanding that always comes with detachment that is the real path.