Belgium enacted an implementing law, the Act of 30th July 2018 on the protection of natural persons with regard to the processing of personal data. This, along with the GDPR, are the key legislative references that relate to your question.
On 5th September 2017 the ECHR judged that it "considers that States should ensure that, when an employer takes measures to monitor employees' communications, these measures are accompanied by adequate and sufficient safeguards against abuse". This case set a precedent relevant to employee monitoring in Belgium.
This is most definitely a data protection or privacy law matter and the DPO should be consulted. If they have to do research, that shouldn't be your concern as that is their job. The company must support the DPO in what they need to do that job, as such is literally written into the GDPR. Furthermore, in this case it would, by my evaluation, be necessary to conduct a Data Protection Impact Assessment (DPIA) for the monitoring activity, and if one has been done, it should document the recommendations and requirements or what is already in place.
You as an individual may also ask a question of the Data Protection Authority:
You might ask them about your position and liability as an employee, but I would be more concerned, if I were you, with verifying that your actions are above board rather than trying to cover your ass just in case they aren't. Do the right thing, even if that means questioning the direction you've been given.
AD logons still identify the person logging on, and may include source IP, which is specifically listed in the GDPR as within the scope of 'personal data'. So while there may be a legitimate need to process such data, it needs to be gone about in the right way.
Actions taken by an employee are taken by the organisation in terms of processing personal data, so whatever you are asked or ordered to do, will be done by the company. If you are being offered no legal justification for doing so, you might document the direction you are given and question it respectfully, pointing out that if found to be unlawful, it is the company that would be in trouble, all while knowing that should there be for example an unfair dismissal, you have a record of who asked you to do what, how you challenged or questioned it, and what was the outcome.