I contacted Apple by phone, I reported the issue and we did some diagnostic etc. They have opened support ticket about this issue and promised to call back after investigating the diagnostics data.
If you have the same issue, please use Apple's feedback formula to report this issue, so they fix it faster.
Feedback - iPad - Apple
Below is the screenshot from my old iPhone 5, which has iOS 10 (10.3.3 14G60). I have one video and one photo on my SD Card, which both are shown on iOS Photos Import screen.
Below is the screenshot from my iPad with iOS 2 (12.0.1 16A404) and it's sowing only image for importing. Video is missing on iOS Photos Import screen.
It's definitely iOS 12 bug, caused by iOS 12, which does not scan folder MP_ROOT as iOS 11 and older versions do.
If I connect Alpha a6000 camera with Apple's Lightning-USB 3 Camera adapter with MTP enabled on camera, I'm even able to import not only regular MP4 format video files but XAVC S video format files too and use them on iMovie. The only drawback is slow USB connection, as it does not support USB3 transfer speed. USB2 is too slow for video files. I bought new iPad Pro 10.5 especially for video editing because of USB3 transfer speed and better performance. At the moment on iOS 12 iPad renders almost useless for video editing, as I'm unable to import fast videos from SD Card.
I'm not the only person having this issue and this for sure affects everyone who uses iPad for fast video editing.