Optimizer failing to load statistic when creating plan in SQL Server 2017 "Calculator failed. Replanning."
For a calculator to fail does not necessarily mean that loading of stats failed.

See Joe Sack's post with the query_optimizer_estimate_cardinality event indicating CSelCalcColumnInInterval CalculatorFailed:

The CSelCalcAscendingKeyFilter Calculator

Regarding the default for any values not in the histogram, Itzik Ben-Gan has a great summary in Seek and You Shall Scan Part II: Ascending Keys, but focuses on numeric data types.