For certain Bech32 strings, deleting or inserting a single character produces a string that is still valid

Overview

For certain Bech32 strings, deleting or inserting just a single character can produce a string that is still valid when tested against the reference implementations.

Implementations Tested

Cases

There appear to be two main cases:

Insertion: if a valid Bech32 string has the suffix p, inserting a single q character immediately before the p will produce another valid Bech32 string.
Deletion: if a valid Bech32 string has the suffix qp, removing the q character will produce another valid Bech32 string.

These production rules can apparently be applied repeatedly, either by deleting many q characters, or by inserting many q characters, producing a valid Bech32 string after each step.

Examples

The string ii2134hk2xmat79tp is a valid Bech32 string, with:

human-readable part ii2
data part 34hk2xm
checksum at79tp

Insertion

By inserting one or more q characters immediately before the final p character, we can produce more strings that are valid Bech32 strings:

ii2134hk2xmat79tqp
ii2134hk2xmat79tqqp
ii2134hk2xmat79tqqqp
ii2134hk2xmat79tqqqqp

Deletion

Conversely, by deleting one or more q characters located immediately before a final p character, we can go the other way. The following are all valid Bech32 strings:

eyg5bsz1l2mrq5ypl40hqqqp
eyg5bsz1l2mrq5ypl40hqqp
eyg5bsz1l2mrq5ypl40hqp
eyg5bsz1l2mrq5ypl40hp

Outstanding Question

It's currently unknown to us whether this is a property of the specification itself, or merely a feature of the reference implementations that we tested.

Background

We ran a suite of property tests against a modified version of the reference implementation, designed to simulate simple user errors such as omitting a character from, or inserting an extra character into, pseudo-randomly generated Bech32 strings.