I have the same issue, but on prduct category pages: Changing of keywords is not stored.
There is another issue on the same page (may be caused by the same failure) with the version 1.0.39 of RankMath:
On product category pages in WooCommerce is the formatting (including images) in Category Description completly lost after saving the category page.
When I load the older RankMath version 1.0.38 from my site backup, this error disappears. So that formatting-lost-error is caused by the newes 1.0.39 RankMath version only.
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