It’s just stress. Heat.
Keep them watered and fed. Some are more likely to have curl than others. Sometimes one could be drier than the others under the mulch,
A bit of shade can help but generally it’s not harmful, just the plants way of showing you it’s a bit stressed. Fix what you can, it will sort itself out easy on pruning, thus isn’t disease and more pruning will add stress not fix it.
KarenO
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