I have myself verified this behaviour with both an Apple USB-C Digital AV Multiport Adapter - and a Kingston Nucleum USB-C hub - both connected via HDMI to a compatible external monitor. With iPadOS 17.4, Stage Manager only becomes available if an external Keyboard and Mouse/Trackpad are connected to iPad before connecting the external monitor.
The behaviour with 17.3.1 and earlier differed - in that Stage Manager was available whether or not an external Keyboard and Mouse/Trackpad were connected. While this arrangement arguably had limited utility without attached Keyboard etc. - as you could not directly interact with Apps displayed - it was still possible to display an App on the external monitor (e.g., streaming video). With iPadOS 17.4, this capability has been lost.
If you have not already done so, you might be well advised to submit feedback and/or a Bug Report via Apple's Product Feedback portal.
Feedback - iPad - Apple
Every report counts - as until sufficient reports are submitted, Apple may not assign resources to investigate and resolve the issue.