It is all about continuous digital (Vedic) data write rate (not the read rate). Just because the Lexat 32 and 64GB UHS II card works, and is recommended (after being tested) does not mean the 128GB 2000x card is going to work, which you have just found out. That is why the 128GB card is Not listed, it does Not work, for what ever reason. Different size cards use different memory components, and may not write data in the same way. There have been numerous discussions previously on the why's and why not of various cards. SanDisk had issues also, a 64 GB Pro card worked in the Pocket camera, but the 32GB version of the same card ISO not! Later, a newer version of the same card did work.
It is all about the testing, BM is doing for you, and recommends which cards to get, to Prevent issues like this -- buying SD cards that will not work. Maybe down the road Lexar will come up with a UHS II 128GB card that will work in the VA 4K, but cirrently, only these Lexar 2000 series UHS II cards Are recommended:
The following SD cards will record at a maximum of 2160p30 inc MXF and are recommended when using HFS+ or exFAT:
Transcend SDXC UHS-II 180MB/s 64GB
Lexar SDXC UHS-II 2000x 300Mb/s 64GB
Lexar SDHC UHS-II 2000x 300Mb/s 32GB
Cheers