Hello! I have recently been having a similar problem, and I haven't found any indication online of anyone having a similar issue until finding this thread here. I am using a Wacom Intuos Pro and an iMac.
The thing is, I purchased a new Wacom Intuos Pro a few weeks ago, which turned out to have a defective USB jack, making its connection to my computer unstable. The pressure sensitivity issues started around that time, and I thought they were a result of the defective tablet. I returned it to Wacom last week, and while I'm waiting for the replacement to arrive I am using my old Wacom tablet. Unfortunately, the pressure sensitivity issues are still happening. After testing out the brush sensitivity in Pixelmator a couple days ago, I found that this issue only happens in Affinity Photo.
As near as I can tell, the issue is identical to what ChristiduToit describes. I have also watched the video you posted, and that matches my issues as well.
I think I may have one additional detail to add on my end. I've noticed that when I make a stroke quickly, touching the pen to the tablet and drawing in one swift motion, the line is consistently thin. If I continue drawing a long line this way while keeping consistent pressure, the size of the brush stroke eventually increases to match the pressure, but slowly. Additionally, if I draw a line by pressing the pen down and then waiting a moment before I first move it, the line will be consistently thick.
If you have any updates on this issue, please let me know. I hope the additional information I had will help.