No, the import is fine.
Here’s what happened:
1. I import the .abr > it gets converted to .artstudio_brushes - this step is completely fine
2. I find brushes I like and either copy/paste them into a seperate group, or add them through “add current brush” menu option - this step also worked fine
At this point I have two copies of the brush, the imported one in the original pack, and the one I copied into a different group.
For a long while, there were no issues with this, but at some point while painting I noticed some brushes in the second group didn’t have any previews, those were the ones I originally uploaded. The same brushes in their own packs are fine, but the one group where I copy them over for organising had some of them borked.
I’m not sure what could have caused it or when precisely it happened, but it’s localised to that one group, perhaps because that’s the one I actively use.
I did have a couple of crashes during that time and maybe my storage was full at some point, I can’t be sure.
To summarise, it’s not an issue with importing or .abr conversion, but at some point a random selection of brushes, all within one pack, had their stamps either corrupted or deleted, and because the brushes were imported, the stamps weren’t located in the stamp library, but wherever they get stored when importing individual brushes.