This is a very common problem. What is causing it?
(In this case I used Offset Loose)
Without the file, I assume you offset the boundary which is a polyline. You could try clipper plugin. Never used it by myself, but every time there’s an offset question it’s the solution.
I notice that it happens when a poly line has control points close together. For example if I have a 100m curve with some control points which are 3mm apart.
Use Simplify Curve and Polyline Collapse components to clean up your polylines then, but still if using polylines, its best to offset with Clipper (I would still clean them up first anyway as a best practice).