[werk_ag]

It is this one, fairly standard issue Chinese clone.

https://www.ebay.co.uk/itm/1PCS-SHT31-SH...1890795555?

which works on the Rev 6 board and the ProPlus boards.

Hi Jim,

Rev 5 and Rev 6 run the same software, so, we can discard a software issue. What seems probable to me, its something related with the I2C bus and its pullup resistors.
Since you say that the sensor works well with a Pro2 Rev 6 board, can you please verify this:
There are any resistors installed at R4 and R5 in your Rev 6 board?
If the Rev 6 is working without those resistors, then perhaps removing R5 and R6 on the Rev 5.xx board will solve the issue with those SHT31.