You are seeing "blistering". I can't really see the material you're using but if it's something like FR1 paper base phenolic it won't take much heat for very long. A typical spec would be 10 seconds. 30 minutes is a very long time.
Most copper-clad laminates are made by large companies and are of rather high quality (in that they meet all the specifications) but the specifications and performance vary significantly between an epoxy-glass laminate like FR4 and the cheaper phenolics. In some cases you can buy FR1 and actually get material that meets FR2 specs because it's not cost-effective for the manufacturer to make both. Which might lead to a surprise if you get FR1 the next time and have not done your design work properly.
There are better grades within the epoxy-glass spectrum too, look for a higher "glass transition temperature" specification.
I have had even prototype boards made in FR2-like materials (actually the lighter color CEM materials) because the end product was going to be made of that (punched, rather than drilled for high volume) and we wanted to evaluate the design with as close as possible to the final materials. Anything heavy or exposed to end-user mechanical forces is a potential point of failure in such a single-sided board since you're depending on the feeble adhesive on a rather brittle material to provide mechanical strength.