Clicks and pops when running UI over Ambience

Question

I have an ambient bed playing a few randomised samples all sat in a blend container. This is routed to it's own bus, sat inside a PRE MASTER bus under the MASTER. When I trigger my UI sounds in the Soundcaster (routed to its own bus nested under the Master) I hear audio artifacts, little clicks.

When I view the meters, the Master audio Bus peaks at -8.2 with Ambience and when I run the UI on top of that it hits -7.5 Short term LUFS is approx 23

From previous scenes and projects I've done, this is nowhere near enough to introduce artifacts. FYI, the UI bus has no compression or limiter on it and all the samples were produced at -3 dB with fade ins and outs.

The transitions on the different samples running the ambience is Xfade power 0.1s. Again, no limiter or compressor on the bus but there is a -6dB Peak Slow Release on the PRE MASTER, just in case...

Any help troubleshooting this is greatly appreciated!