In the latest version of Affinity Photo, 1.7.2 from Apple Store, there appears a strange bug. ISO values are incorrect in many cases, but not all. For example, ISO 400 in a TIF file, will be shown in Affinity's EXIF tool as 800 and will be written as such on the exported jpg. ISO 125 will be shown as 72. ISO 640 will appear as 1987 and ISO 1000 as 4935. This occurs with TIF files coming from Canon's DPP 4, on both my Canon G7X mark 11 files and my Canon 77D files. The ISO on the incoming TIF files matches that of the CR2 files. Other programs, such as Xnview show the ISO on the TIF files correctly.
In some instances, particularly those with ISO 100, the error does not seem to occur.