Do employers allow employees to review materials relevant for the job during the working hours?
Strictly answering the question, some do, some do not.
I'm going to infer there's an XY problem at hand here: "How much time should be spent on self-learning?" which again depends on your manager/employer, but if they are a competent manager of a tech company, they should expect a certain amount to be allocated to pure research, depending on the exact job role.
You specifically mention "machine learning engineer". This could be:
- Software engineer - you are taking pieces developed by other software engineers and data engineers and putting them into an application
- Data engineer - taking researched algorithms and putting them into a pipeline for use in production
- Data scientist / analyst - you have reams of data you need to make sense of
- Researcher - researching and developing novel algorithms
- Senior engineer - like the above, but you also have some engineers/scientists
- PI (primary investigator) - you are orchestrating others to do the above
As you go up this list, you can and should be doing more pure research to stay on top of advancements, and implementing challenging mathematical work into code.