The root cause of that behaviour is the call to {{pclass#fromString}} on https://github.com/xwiki/xwiki-platform/blob/masterxwiki-platform-14.10.17/xwiki-platform-core/xwiki-platform-oldcore/src/main/java/com/xpn/xwiki/objects/BaseObject.java#L348 since this method systematically creates a new property: since this new property has no value, setting its value makes it dirty and so the owner document metadata dirty flag is also updated.
We probably cannot get rid of this call to {{#fromString}} directly, as it provides various implementation of handling the String parameter. However, we can probably provide a new API to allow reusing an existing property, instead of creating a new one systematically.
Now note that fixing that bug might have unexpected impacts on existing extensions: I discovered that some features of CR were working by accident because of it.
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