It seems you haven't followed any of the recent developments regarding the matter then. Originally they were supposed to strip Usyk of the IBF title just two weeks after he beat Fury for undisputed. What happened instead was that Team Usyk requested an exemption from having to fight his mandatory at the time, Hrgovic, next since he was already contractually obligated to rematch Fury.
Not only did the IBF not strip Usyk they created an interim title for Dubois vs Hrgovic. Dubois won that fight and just became Usyk's mandatory. The IBF had already given Usyk permission to defend the title against Fury in the rematch which isn't until December. Usyk chose to vacate it voluntarily which was quite unexpected. Notice in the video he said it was a "present" to Joshua & Dubois. He wasn't forced to vacate.
Why was Oleksandr Usyk allowed to keep his IBF belt?