Mbed Studio 1.4.1 has just been released on all supported platforms. It no longer passes connect_mode=under-reset for that target. Please take a look if it works correctly for you now.
I’m working with Mbed Studio 1.4.1 and a J-Link EDU, and I’m facing the “stuck on erasing” problem. I do not have a problem flashing (or erasing) with either Keil or VS Code with PlatformIO.