The Kunena team has announce the arrival of Kunena 7.0.4 [K 7.0.4] in stable which is now available for download as a native Joomla extension for J! 5.4.x/6.0.x. This version addresses most of the issues that were discovered in K 6.2 / K 6.3 / K 6.4 and issues discovered during the last development stages of K 7.0
I tested this issue with php 7.
On my life site I only upgraded from php 5.? to 7 recently.
Maybe this issue was raised since then.
I did not check how the block size used during the copy is determined. This can be different by using an other PHP version.
It comes down that in my opinion there is no need to check the image orientation while copying the file. This is only useful after the file is completely copied.