See U8G2 FAQ on Github. Here is the pertinent section (First question) :
Q: Why does my xxx_SW_I2C() device not work with my other I2C devices?
A: SW_I2C emulates I2C with digitalWrite(), which will have a conflict with other
I2C devices at the same pins. There are two options: (A) use xxx_HW_I2C() or
(B) use different pins with xxx_SW_I2C()
The RTC and OLED are both on the same I2C bus.
If I switch the U8G2 OLED device to
U8X8_SSD1306_128X64_NONAME_HW_I2C u8x8(/* reset=*/ U8X8_PIN_NONE);
both the display and the OLED work on my expansion board with the XIAO (not BLE.)