The convenience of a science oven is long ubiquitous; so too the issue of uneven heating. The problem is confused convection.
Old-style heating had hotness coming from below a container of fluid. Via convection, as liquid warms at the bottom it becomes less dense and moves to the top, allowing a cooler section of the liquid to contact the heating source. This ultimately results in a uniform temperature throughout.
Inside a science oven, the electric (microwave) field acting as a heat source works everywhere, including the glass container. Vertical convection is thwarted. The liquid at the top of the container ends up being much hotter than the liquid at the bottom.
Solids don’t undergo convection. As with food on the stovetop, stirring is needed to evenly heat microwaved food.
References:
Peiyang Zhao et al, “Multiphysics analysis for unusual heat convection in microwave heating liquid,” AIP Advances (4 August 2020).
“The problem with microwaving tea,” ScienceDaily (4 August 2020).