I just discovered a bug occurring when immediate dynamics, with their beat-relative position property set to Before (either explicitly or implicitly), are positioned in a system of a different staff size showing on the previous system as a consequence of its Beat-relative position: instead of adapting its size to the staff size of the system it belongs to “physically”, it adapts to the staff size of the system it belongs to rhythmically. This screenshot shows what I mean:
Yes, we know about this, and it’s something we plan to address at some point in the future. For the time being, as I expect you have already worked out, if you move the dynamic back a little way so that it is attached rhythmically at the end of the previous system, you can achieve the result you’re looking for; or you could indeed use the Custom scale property to adjust its visual size.
Thanks for your suggestions! In the end, I ended up using the Custom scale property, set to the ratio between the two different space sizes.