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Sorry for the long post, but since this is a bit confusing for me, I thought I should give the best context I could :dry:

TL;DR: seemingly well-cared for serissa got browned from possible underwatering and never recovered new leaves on it's completeness, but it's still budding at some small spots and its roots. What should I do to improve its condition? Is it dormant? Should I do some prune or change in its routines?

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I have three Serissas on the same (deep) pot, where I'm trying to grow them to make a small groove some years from now. Two of them are quite happy, in their ways, but one of the normal ones got underwatered a couple of months ago (I think I had some trouble with my auto-watering system).
Most of its leaves browned, but almost none of them felt, not even passing my fingers gently through it. I got worried, but one of its sides still slowly trickles in new leaves... Which then got dry, and I started checking the soil for dryness and reinforced watering when I noticed that. This week I also noticed there was a lot of pesky buds growing from its roots, which I pruned today (something usual on the two normal serissas, as far as I know?)

Here are my beginner theories on what might (not) be happening, and how they're being cared for:
- sunlight: I would doubt that's an issue, even though they only get direct light for half of the day (they're outside of a west-facing window). The other normal serissa "sister" is really great, with lots of flowers (not getting much bigger, tho), and the Chinese one is growing very well with a couple of flowers every now and then.
- fertilizer: they all get weekly sprayed with flower fertilizer, and monthly with sulphocalcium (lime sulphur?) to avoid fungus/mites and the likes. Every now and then I add some organic fertilizer to the soil as well, and treat them all the same. The sprays are always during the afternoon, hardly at night, times when there's no direct sunlight on them.
- pruning: I tend to let them grow as they want before deciding on shapes, just doing some light pruning to avoid getting "too messy".
- water: as I mentioned before, it was probably underwatered for a week, but that was early July and it's been fixed(?) ever since. They all get water every 2 days, and I add extra water in between when I see the soil seems too dry. Lastly, they also get sprayed almost daily, except for that 5-day period, since I had travelled, and when they get spray fertilizer.
- I also can't see any sort of pest or disease on it, unless it's very hidden or under the soil. But then, if it was under the soil... it wouldn't grow buds from its roots, would it?

Location/climate context: I live in Rio de Janeiro, but for the last few months we're having an usually long winter. These three plants were grown in a more "temperate" climate of the south of the country (Santa Catarina), and are living here in Rio for almost a year now (arrived at the end of spring, last November). They faced a moderate/hot summer and were fine. And this winter we didn't have a strong variation (as it's usual and could drive the serissas nuts), it's been cold for most of the time since the start of the winter - averages of 22~24C/71~73F in July~September ( ).

More context on them: the right one, full of flowers, is a doubled flower variety, around 4 years now. The middle one was the smaller, from a Chinese variety and simple flowers, 3yo now and the taller/fuller of them. And the browned one, at the left, is the older, with pinkish flowers and 5 years. I'm attaching a catalog pic of the browned one, to compare how it arrived with how it is now (sorry for the little car on it, I guess Harry Potter fans will understand it, if any haha)

So, the question is: should I do something about the browned one? Could this be some dormancy it entered after the lack of water and the start of winter? A hard prune on all those branches with browned leaves? Try to clear them by pinching? Leave them be? Fix something else in its care? Care for this one in a different way from the others?