Hey Vito,
The fact that the same files play back fine in 26.2.2 but not in 26.3 doesn't sound like a problem with your media. It sounds like hardware decoding is falling back to the CPU.
On your laptop, HEVC 10 bit 4:2:2 is decoded by the Intel iGPU (Quick Sync). Your RTX 3070 can't decode 4:2:2 at all. So if that Intel path drops out, your i7 has to do it on its own, and that's exactly when you get this kind of stutter.
Stuff I'd check:
Play a clip and open Task Manager, go to Performance, look at the GPU and specifically the Video Decode engine, for both GPUs. If the Intel one stays at 0 percent, you've pretty much found it.
Then go to Preferences, Media, and make sure Enable hardware accelerated decoding is still on and that your Intel GPU is actually listed there. Toggling it off and back on after a restart sometimes fixes it.
Your Intel driver is another suspect. On MSI laptops it's often years out of date because people only update the NVIDIA driver. Grab the latest Iris Xe driver straight from Intel, not from MSI. Also check in the BIOS that the iGPU isn't disabled and that you're not running in Discrete GPU only mode, because that kills Quick Sync completely.
Other than that, use the NVIDIA Studio driver instead of Game Ready, and clear your media cache through Preferences, Media Cache, Delete unused.
Then do a proper A/B test. New empty project, one clip, playback resolution on Full, no effects, in both 26.2.2 and 26.3.2. That way you know for sure it isn't your sequence settings.
If it still happens, I'd just stay on 26.2.2 for now. You can reinstall it through Creative Cloud, under Premiere Pro, Other versions. And file a bug report with Adobe, include your System Info and ideally a sample clip, because this smells like a regression in 26.3.
If that Intel decode engine is active in 26.2.2 but idle in 26.3, that's your answer, and that's the main thing to put in the bug report.
Good luck with it.
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