Temporary Workaround
I've made this Keyboard Maestro macro which executes four Logic commands with one shortcut key. I've used F19 on my extended keyboard, but almost any key can be used.
Making a Marquee selection in Logic deselects the region, so I've add Select All and Deselect All commands.
A fully working 30 day trial of Keyboard Maestro is available. Hopefully Apple will fix the problem before the trial period ends. Keyboard Maestro is $36.
https://www.keyboardmaestro.com/main/
Here's the zipped macro file which can be imported into Keyboard Maestro.
The macro will be disabled by default, so click the "Triggered by. . ." tick box - see top left of first image in this post.
Split Region at Marquee Selection - Workaround.kmmacros.zip
Slower computers - pause commands can be added between the four commands e.g.