For a few documents I use the ‘Flatten Selection to Bitmap’ a lot and it doesn’t work anymore in Sketch 2025.3. When I scale down an image in a file with a lot of other images or other objects and select ‘Flatten Selection to Bitmap’ from the Layers-menu, Sketch seems to add a huge space to the selection. Sometimes the image just disappears, but in most cases I got a beachball and the application just crashes.
Could reproduce this multiple times.
Hi Emiel. 
Thanks for reporting this issue. It appears this one slipped under the radar in this release. I’ll file an issue for this to be fixed ASAP. Apologies for the inconvenience in the meantime. 
Ah, good to know. I just tried it again in a file with many images… result: I had to force quit most apps I had open (Mail, InDesign, Safari, DuckDuckGo etc.)
The issue appears to be that when making any selection, and then choosing Layer > Flatten Selection to Bitmap, only the selected layers will become part of the bitmap, but the boundaries of the bitmap will take into account the area of EVERYTHING on the Canvas. And in very large complex documents, that can be a very big area to cover, hence the application stopping up.
In the meantime, as a workaround, what you can do is copy the relevant layers into a new Page or Document – ie: where there is nothing else on the Canvas – perform the Flatten action there, and then bring that resulting bitmap layer back to whee you need it. Not ideal, I realise, but it will work in a pinch.