When I try to save an entity after denormalization I get this error: Call to a member function getProcessedText() on string in core/modules/text/src/TextProcessed.php line 43. This happens because denormalized entity has the ''processed" property as a string eg. <p>Lorem Ipsum.</p>, not as FilterProcessResult object.
The issue seems to be a followup for #2626924: Include processed text in normalizations: "text" field type's "processed" computed property should be non-internal and carry cacheability metadata.
I've found this issue while working on making Relaxed module compatible with Drupal 8.5.x and this is the last blocker.
Attached a failing test and also a fix.