Currently the Primo VE drool editor will not display error messages in some situations where the rules crash and fail.
Instead the Discovery Administrator flows through the process of updates believing that their changes to the rules are correct and have taken effect, only to find perhaps much later after reindexing request that they have not.
Actually what happens is that if the rules fail, there is no error shown, and the entire ruleset fails back to the out-of-the-box rules.
This is a considerable hindrance given the situation of no option to test display rules.

Ex Libris advised in a case: “When there is any error during building the customized display of Primo VE record, the system fallbacks to OTB rules. It may also depend on the context of specific error, but usually all custom Primo VE fields are then not being displayed. The Rules editor has built-in validations, also Apply rules can report additional detected issues in Normalization rules, but it is not bulletproof. Most such issues are caused by incorrect regular expressions applied on specific data, but here it seems to be caused by specific syntax of the drool.”

The decision was to document the particular scenario of what will cause the OR operator to fail, rather than fixing the situation overall: Configuring Normalization Rules for Display and Local Fields - Ex Libris Knowledge Center

Ex Libris deems this to be an enhancement request otherwise, to improve the messaging to show a user when their rules have failed and reverted.