olikraus
Probably you need to use one of the SSD1309 constructors: https://github.com/olikraus/u8g2/wiki/u8g2setupcpp#ssd1309-128x64_noname0
I suggest to try with "U8G2_SSD1309_128X64_NONAME0_F_SW_I2C" or "U8G2_SSD1309_128X64_NONAME2_F_SW_I2C". Provide the pin numbers where your display is connected. This constructor should be compatible with the u8g constructor.
You can slow down u8g2 as mentioned in https://github.com/olikraus/u8g2/issues/1392
hmm... what confuses me is, that a constructor "U8G2_ST75256_JLX256160_F_1_4W_HW_SPI" does not exist in u8g2. How can your code work?
ah, ok... but yes, the upper lines look strange. Maybe you can retest this with some other examples like the FlipMode.ino Will the strange lines always appear at the same...
Anything else than the standard Arduino API functions is difficult for me. Of course it is doable by re-implementing the low level functions: https://github.com/olikraus/u8g2/wiki/Porting-to-new-MCU-platform