This is apple's fault, and I encourage you to pressure them, not affinity, here: http://feedbackassistant.apple.com
As a developer myself, and a former apple employee from the Steve Jobs era, it is really tiresome talking to them because they no longer have truly competent programmers and are constantly breaking core OS functions and never fixing them. The current apple team needs to be heavily pressured by submitting many bug reports by many people. Even for trivial things that can be solved in one day and even when the exact code fix is provided to them, there are trivial problems like this that last for years across OS releases. I assure you, even though this sounds extreme, it is truly the situation. As developers, we need the users to vocalize the complaints more loudly than is rational, or they just act like it isn't important. They don't hold the same credo as 10 years ago, and they don't care unless we really pressure them. This is something that has gotten worse every year in the last decade, and I can not encourage you enough to submit bug reports to http://feedbackassistant.apple.com
To explain why this happens:
The first data that comes from the stylus through the apple library is hard coded as 0.33
Because of this, if the developer does not deal with that, it will cause the problem we see in affinity photo on the iPad.
To elaborate this further, the real pressure is always coming in later than the position. They will rationalize this by saying that it will be able to give a more accurate reading in the next 8 milliseconds - however, they neglected to write that code and so this doesn't ever happen. The 0.33 hard coded pressure at the start of the stroke is never ever updated. Leaving the developer to come up with their own way of coping with this bad data.
The way I deal with this, and how every other app deals with it, is by not showing the first data as 0.33 - either by making it much smaller or just not showing the first few points at all. Affinity can perform this with a few lines of code as an interim fix, but I still encourage everyone who really cares about drawing on the iPad to flood apple with bug reports. Be sure to also submit reports on the other issues, such as the intermittent stroke disconnects, the intermittent lag, and processor buzzing (which is amplified with a connected cable and certain tablet stand materials). These are not things a third party developer can fix, and they have gotten worse on the latest iPad.