Cropping in lightroom

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"This is also a huge problem after straightening an image"

That provided the clue. LR is operating consistently, as-designed.

If you squint at your unfortunately low-resolution first screen recording, the crop angle is not 0, it's 0.01. As you try to enlarge by dragging the upper-left corner, the lower-left corner is expanding into the bottom edge of the image.

A larger crop angle makes it easier to understand what's going on.  Start with this crop, with angle = 5.00:

When you drag a corner of a crop with the aspect ratio locked, LR keeps the diagonally opposite corner fixed. So when you drag the upper-left corner, the crop expands by moving the left edge and top edge. Soon enough, the bottom-left corner bumps into the bottom of the photo:

The same thing happens when the crop angle is 0.01 and the bottom crop edge starts out very close to the bottom of the photo -- it's just much harder to see.

If you set the crop angle exactly to 0.00 by double-clicking on "Angle", then you'll be able to fully enlarge the crop.

(I learned about the subtleties of this behavior while implementing the Any Crop plugin.)

You can use the standard Option (macOS) or Alt (Windows) modifier key to scale the crop rectangle from the center. In the demo below, I first drag from the bottom right corner normally. But that cuts off the truck, so next time I hold down the Option key as I drag, and it scales down from the center. The third time, after Option-dragging to crop from center, I then release the Option key without releasing the mouse button, so that when I drag back down, this time it extends the small center crop down to the bottom right corner, to include the truck.

What this shows is that by using the Option/Alt key, you can crop to the center, or two opposite corners, from the same crop rectangle handle. If you drag a side handle, you can crop to opposite sides (left/right, top/bottom).

If the aspect ratio was not locked in Crop tool options, you could also hold down Shift to lock aspect ratio as you drag.

These modifier keys have been standard for many years: The Option/Alt key scales from the center, and Shift toggles aspect ratio lock. After you learn them, you’ll find they also work for general drag-scaling in Photoshop and many other applications and in the OS itself. For example, if you want to resize a Finder desktop window from the center, Option-dragging a window corner will do that.