Assuming you are using the latest beta 2.8.2.41, and not 2.8.1:
Yes, the product serial number needs to be shortened if it is longer than 32 chars (matter side restriction). Your dimmers have unusually long serial numbers and are therefore affected. In the next beta, the serial number will be truncated in the middle inserting an ellipisi, to catch that edge case better than by truncating at the end.
However, the spec does not give (or require) any guarantee about the serial number's uniqueness. It's very much possible to encounter devices which do not even have a serial number. So while the serial number might be a hint for controllers when they try to re-identify devices, it's not more than that, a hint, and controllers should definitely not mess up when that hint is not unique enough.
What the spec does say something about is the endpoint id (which must remain stable for the same bridged device) and the UniqueID attribute, which is what it sounds like, an unique ID meant for re-identifying devices, which is generated from the dSUID in the case of the plan44 matter bridge, so is unique for your lights.
HA is not the only controller that sometimes messes up when trying to re-identify devices, because it's up to the controller's maker what strategy to use, and there's no guidance in the specs about best practices. But clearly, the uniqueID (mandatory since 1.4, optional before) should be prioritized over using the serial number.
Note also that the uniqueID is not stable over matter bridge factory resets (in fact the spec demands that it should not be).
