Fix mask treatment for lm covariance matrix update

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.52%. Comparing base (30af5c6) to head (e1b1342).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #176 +/- ## ======================================= Coverage 79.52% 79.52% ======================================= Files 91 91 Lines 7616 7616 ======================================= Hits 6057 6057 Misses 1559 1559 ```

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[ConnorStoneAstro]

Closes #174