Hi, I think I have identified the root cause and found a workaround for it
The problem is that the values are interpreted as signed int8 and not unsigned Byte. I saw this as I tested the receive function and found that the value 128 (1000 0000) was displayed as "---" (no number) and 129 was displayed as -127 on the EV3. This means that the EV3 interprets the values as 2's complement and not as unsigned Byte.
So if you want to send a value >127, then you need to send a negative number. Unfortunately the I2C block doesn't allow the input of negative values, but it is possible to put the negative number into a contact or variable block and connect this to the I2C input.
One issue remains: the 2's complement of 1000 0000 is "-0" which you can't enter in EV3. But I found that you can receive this value via I2C, store it into a variable block and then send it back via I2C.
To test for a recieved "-0" value it works to use the range test "-127 to 127" where it is not included (out of range). Whith this you can convert between the unsigned Byte and signed int8 values.
Not sure if this helps to fix the issues with the I2C block (to interpret the Byte correctly), but I hope it helps other users to workaround this limitation and use the full range of values (except maybe the "-0" case).
To convert an unsigned Byte into a singed int, you need to test if the value is above 128 and if yes substract 256. Example: Byte 130 = int8 130-256 = int8 -126. The issue remains with the value 128 which can't be converted to a "-0" in EV3 (at least I have no idea how).
In the other direction you just add +256 if the received value is <0.