I'm an art streamer on Twitch who's spent the past 12 months using Affinity Photo for my digital art. In any software I use, whenever I encounter a bug on stream I ask my moderators to "clip it" as I demonstrate the steps to reproduce the bug. Here's a clip where the flow rate of the watercolour brush is affected by zooming on the canvas. In the brush settings don't reflect any change. I've mentioned a workaround at the end of this clip.
A summary of what's happening in the clip - I'm using one of the watercolour brushes whose default Flow Rate is 5%, though I've increased that to 20% for a harder edge. It appears that after zooming, the bug may be reverting the brush to its default flow rate. If I deliberately resize the brush (I use Alt + middlemouse + drag to resize brush), my custom brush setting returns.
Windows 10 - i9-7900X @ 3.30GHz 3.31GHz - RAM 32GB - NVIDIA GeForce GTX 1080 Ti (x2) - Wacom Intuos Pro 4
Cheers!