display – Thunderbolt monitor running at 30Hz when second monitor connected

I have a macbook pro (15 inch, 2018, Radeon Pro 560X) to which I plug 2 external monitors: a LG 32UL950 4K over thunderbolt & an old ASUS full HD over HDMI.
The problem is when the ASUS is connected then the LG is only running at 30Hz.
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When connected alone it’s fine at 60Hz.
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This machine is supposed to support 4 4K@60Hz screens. Any idea how to fix this?
(I can’t confirm it but I think this issue started with one of the latest macOS updates, 11.3 or 11.4)

EDIT: Display preferences
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