During performance tests on IO-Link we were seeing frameloss on 400µs cycletime. Sometimes parallel to communication we need on low priority store data to eeprom. 

It turns out that I2C is all the time during transfer in ISR. By this nothing else can be processed. In our case this took around 2ms which is too long if we need to process data within 400µs on the IO-Link protocol. 

Look like the issue is root caused in function I2CBufferStatus(). Here are the bitshifts of the readings are not considered. by this the results are wrong and not correctly processed.