Updating Products not consistent through API

It seems updating products through the API does not always work.

Is there a cache that needs to be cleared each time, like the transient_cache?

If I set the sale_price it sometimes works and other times does not, and un-setting the sale_price seems not to be possible. The API also does not always show the update, like it is cached. What are the rules for this?

Set sale price

{
    "regular_price": "49.99",
    "sale_price": "49.50" 
}

Un-set sale price

{
    "regular_price": "49.99",
    "sale_price": "" 
}

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