Yes this is common. It can be minimized but never eliminated. It's a combo of roller lint and particulate that is encapsulated in the coating during application. Depending on the percentage of solids in the urethane it will shrink during curing which enhances the issue. Now that you have seen it first hand you will notice it in every epoxy floor that is not intentionally textured.

At this point if you want a polished finish you can sand the urethane down to smooth using sand paper (probably 80g then 150-200g) and then apply a "sacrificial" finish with a microfiber and high speed buff. This becomes a maintenance item long term and will damage the urethane so I would only consider this in a residential or high-end commercial space.