Our 5 months old kitten struggles with panting spells during zoomies and the heatwave, we’ve been to the vet but still have questions

We adopted two kittens in early Spring.
They are otherwise calm, social, eat well, cuddle and purr, but since she was about 2-3 months old we noticed that only one of them had short bursts of panting (sounds like labored breathing, mouth open, only a few seconds at a time) while doing intense exercise. She is also the most lanky out of the two (had a fast growth spurt), and this one was bottle fed when tiny.

At about 4 1/2 months old, during a heatwave (they are strictly indoor cats), she had more concerning panting episodes after mildly intense zoomies, panting with her tongue out for 1-2 minutes at a time. Running, panting, resting a few seconds, and repeat.

We rushed to the vet, who listened to her heart (was fine) and lungs (heard a faint whoosh). She sent us to get an X-Ray, which came back clear with no defects visible. She gave us a course of treatment for a week consisting of Hedylon pills (a corticosteroid, 1/2 pill for the first 3 days, then 1/4 pill for the next 4 days).

We were thrilled that she didn’t have more episodes during that week, but now she got to a baseline of being fine most of the time then still audibly panting in bouts of about 10 seconds after most times she runs a little more.

The other kitten doesn’t do this at all.

They are up to date with all anti-parasite medication and vaccines. We asked the vet if we should come for another checkup but she said it doesn’t seem concerning and to come again in a few weeks for their regular anti-parasite pills.