Mutation testing finds the assertion you did not write

Ninety-four percent coverage and a suite that passes with the comparison inverted, because coverage measures execution rather than verification.

$ vendor/bin/infection --min-msi=60 --threads=4

Mutation Score Indicator (MSI): 61%
Mutation Code Coverage: 88%
Covered Code MSI: 69%

# 412 mutants: 251 killed, 118 escaped, 43 not covered

The three numbers say different things: MSI is over all mutants, covered-code MSI is over the ones the tests reach, and the gap between them is the untested code. Covered-code MSI is the one to watch, because it measures whether the tests that exist actually assert anything. The tool changes the source, runs the suite and reverts — so it needs a clean working tree and is emphatically not something to run against a shared database.